WO2008117234A1

WO2008117234A1 - Digital interface for connecting audio-visual devices

Info

Publication number: WO2008117234A1
Application number: PCT/IB2008/051099
Authority: WO
Inventors: Engelbertus Van Willigen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-03-28
Filing date: 2008-03-25
Publication date: 2008-10-02

Abstract

A digital display interface connects a first audio-visual device to a second audio-visual device. The interface is defined by an industry standard and comprises a set of lines. The interface comprises a first transport channel for carrying video data in uncompressed form and a second transport channel for carrying video data in compressed form. The second transport channel uses a subset of lines of the interface which are reserved in the industry standard for a different use. Both an uncompressed form of the video data (e.g. a channel-coded format such as HDMI raw video) and a compressed form of the video data (e.g. a source-coded format such as MPEG video) can be carried over an existing interface, without the need to define a new interface or connectors. The second transport channel can support bi-directional transport of compressed video data across the channel and can comprise a Universal Serial Bus connection.

Description

Digital interface for connecting audio-visual devices

FIELD OF THE INVENTION

This invention relates to a digital display interface for connecting audio-visual (AV) devices.

BACKGROUND TO THE INVENTION

Consumer Electronics (CE) devices, such as set-top boxes, optical disc players and digital recorders typically have one or more Digital Display Interfaces (DDIs) for the transport of digital audio-visual (AV) content between the device and a display. Transporting AV data in this manner, in the digital domain, overcomes the loss in quality which results from the use of analog interfaces such as component video or composite video. Digital Display Interfaces carry uncompressed (raw) video data and may also carry audio data. A source device decompresses/decodes the data from a format used during transmission, such as MPEG2, MPEG4, or other compressed video format, and outputs a stream of uncompressed (raw) video data. As a Digital Display Interface is intended to connect a source AV device to a display, it only supports video in an uncompressed form. If the HDMI interface is connected to a recording device, this requires the recording device to compress (code) the uncompressed signal before storing the data. It is not always possible for the recording device to perform this compression due to the required processing power, bandwidth and copy protection across the digital interface. One solution to this problem is to provide an analog interface, or an additional digital interface, between a source device and a recording device in addition to the digital display interface between the source device and display. This requires additional cabling and an AV device requires further connectors on the housing of the device.

US 6,985,530 describes an integrated receiver decoder for simultaneously transmitting compressed and uncompressed signals. This allows a set-top box to connect to a display device and to a recording device using a digital interface. However, no details are provided of the interface itself.

There is currently already a range of DDI standards which include High- Definition Multimedia Interface (HDMI), Digital Visual Interface (DVI), Unified Display Interface (UDI) and DisplayPort. It is undesirable for a manufacturer of an AV device to support all of the possible interfaces as it requires additional hardware and software on the AV devices, which increases the cost of the device. Defining a new digital interface to support both compressed and uncompressed data will cause further confusion among consumers, and will further increase the cost of the device. Furthermore, if the new interface is provided on a device in preference to other legacy interfaces there is a risk that the device will be incompatible with other devices.

The present invention seeks to provide an alternative way of interfacing AV devices.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides a digital display interface part for use in a first audio-visual device, the interface being for connecting the first audio-visual device to a second audio-visual device, the interface being defined by an industry standard and comprising a set of lines, the interface part comprising: first transport channel for carrying video data in uncompressed form; and, second transport channel for carrying video data in compressed form, wherein the second transport channel uses a subset of lines of the interface which are reserved in the standard for a different use. An advantage of this aspect of the invention is that both an uncompressed form of the video data (e.g. a channel-coded format such as HDMI raw video) and a compressed form of the video data (e.g. a source-coded format such as MPEG video) can be carried over an existing interface, without the need to define a new interface or connector. This avoids the need for device manufacturers to provide a further, new, connector on their products which is incompatible with existing connectors. The use of the subset of lines to support a second transport channel can be proprietary or it can be standardized in a future revision of the standard defining the interface.

It will be understood that a complete interface between a first audio-visual device and a second audio-visual device will typically comprise a first interface part in the first audio-visual device, a second interface part in the second audio-visual device and a cable which is terminated with suitable connectors for mating with a connector on each of the first and second interface parts.

The digital display interface can comprise a plurality of the first transport channels, for carrying uncompressed video data and audio data. Preferably, the second transport channel is also capable of carrying audiovisual data, i.e. a combination of audio and visual data (e.g. video and an accompanying audio soundtrack) in compressed form. Other, non AV, data can also be carried by the second transport channel. Preferably, the second transport channel supports bi-directional transport of compressed video data across the channel. This allows devices to be connected in new configurations. Some existing digital display interfaces, such as HDMI, require a display to be connected at the top of a device hierarchy. The connection topology resembles an inverted tree, with the display at the top, and this requires that all AV data flows towards the display. This limits the position where a recording device can be connected. By supporting bidirectional transport of compressed video data across the second transport channel, a recording device can be connected to a display and can receive compressed video data from the display, which allows more flexibility in how devices are connected. Various other new connection topologies are possible which carry compressed video data in a direction which flows away from the display.

Preferably, the interface comprises control logic arranged to determine if the second device supports use of the second transport channel. This ensures that the alternative use of the subset of lines is not made with incompatible devices, such as legacy devices. Preferably, a Universal Serial Bus connection is supported with the second device using the subset of lines of the interface. The second transport channel is carried over the Universal Serial Bus connection. USB is a well-defined standard which has a high data rate suitable for carrying compressed video and audio data. The use of another existing standard to implement the second transport channel reduces the cost of the additional parts required to support the second transport channel. The USB connection can also be used to carry any non- AV data which is not accommodated by the standard defining the interface.

In a preferred embodiment of the invention, such as where the digital display interface is HDMI, one of the subset of lines is reserved in the standard for carrying control data. Control logic is arranged to bridge control data, which would normally use that control data line, over the second transport channel. In this way, the compressed video data and the existing control data can both be carried and the new interface part is backwards compatible with legacy versions of the interface.

In a preferred embodiment of the invention, the digital display interface is HDMI and the subset of lines is a pair of lines. One of the pair of lines of the interface is reserved in the standard for carrying Consumer Electronics Control (CEC) data (line 13, HDMI Type A connector) and the other of the pair of lines is reserved in the standard as a not connected line (line 14, HDMI Type A connector). An advantage of using the CEC line is that the functionality associated with this function is typically implemented using a microcontroller, rather than the specialized integrated circuits used for the first transport channel(s). The new functions can therefore be added with low additional cost.

Another aspect of the present invention provides an audio-visual (AV) device comprising the interface.

A further aspect of the present invention provides a method of distributing audio-visual data between a first audio-visual device and a second audio-visual device using a digital display interface defined by an industry standard, the interface comprising a set of lines, the interface comprising a first transport channel for carrying video data in uncompressed form, the method comprising, at the first audio-visual device: sending or receiving audio-visual data in a compressed form using a second transport channel of the interface, wherein the second transport channel uses a subset of lines of the interface which are reserved in the standard for a different use.

The audio-visual data sent or received in compressed form using the second transport channel of the interface can be video data, audio data, or a combination of audio and video data.

The interface can be used to carry both an uncompressed form and a compressed form of the same video content. Alternatively, the interface can be used to carry audio-visual data in compressed form which represents different content compared to the video data sent or received in uncompressed form. The direction of transmission of the uncompressed and compressed forms of the content can be different.

The functionality described here can be implemented in software, hardware or a combination of these. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. Accordingly, another aspect of the invention provides software for implementing the method. The software may be stored on an electronic memory device, hard disk, optical disk or other machine- readable storage medium. The software may be delivered as a computer program product on a machine-readable carrier or it may be downloaded to the AV device via a network connection. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 shows a way of using a digital display interface according to an embodiment of the present invention to connect a source of AV content to a display;

Fig. 2 shows a way of using a digital display interface according to an embodiment of the present invention to connect a source of AV content, a recording device and a display;

Fig. 3 shows the same devices as shown in Fig. 2 connected in a different order;

Fig. 4 shows an alternative way of using a digital display interface according to an embodiment of the present invention to connect a source of AV content, a recording device and a display;

Fig. 5 shows a conventional pin arrangement of an HDMI Type A connector; Fig. 6 shows a detailed schematic of an HDMI interface according to an embodiment of the present invention;

Fig. 7 shows a further embodiment of the invention where a compressed stream and an uncompressed stream carry different audio-visual content.

DESCRIPTION OF PREFERRED EMBODIMENTS

Figs. 1 to 4 show four example scenarios in which the invention can be used. In Fig. 1, an AV device 10 which is capable of providing a source of digital AV content is connected to a display 20 via a digital display interface (DDI) which comprises connector parts 11, 21 and a connecting cable 15. The source device 10 can be a device which can retrieve digital AV content from a store 12, such as a hard disk, a solid state memory or an optical disc or any other fixed or removable storage medium or device. AV content is stored on store 12 in a compressed form to reduce storage requirements. The compression coding scheme can be MPEG2, MPEG4 or any other compression scheme. Examples of device 10 include a personal video recorder (PVR), an optical disc player such as a DVD player, an HD-DVD player or a Blu-ray player. Alternatively, device 10 can take the form of a set-top box which receives content from a distribution channel (such as a broadcast channel or broadband network or other type of network (e.g. home network)) in compressed form. The video portion of the compressed content which is retrieved from store 12, or which is received from a source, is decompressed (decoded) by a decompression decoder 14. Typically, following decompression, the video portion of the decompressed content is pixel coded, i.e. coded on a per-pixel basis (e.g. a digital value for each of the red, green and blue components of a pixel). Depending on the digital display interface, the audio portion of the content will either be decompressed into e.g. Linear Pulse Code Modulated (L-PCM) format or will be sent in compressed form. HDMI 1.3 allows audio to be sent in L-PCM form or in IEC 61937 compressed audio format.

In accordance with the invention, the digital display interface 11, 15, 21 carries both the uncompressed video data, as decompressed by decoder 14, and a compressed version of the same content which has not undergone decompression. In Fig. 1 the label 'C refers to the compressed form of the content and the label 'UC refers to the uncompressed (decompressed) form of the content. The compressed form of the content (C) can comprise compressed audio-visual data, i.e. compressed video and compressed audio. The audio may represent a soundtrack accompanying the video content. The uncompressed video data can optionally be encoded for transport over the interface 11, 15, 21 (e.g. line coded, differentially coded and/or packetized) or can be converted to adapt the video content to a form which is compatible with the display 20. For example, video content having a resolution of 72Op may be up scaled to 108Op. However, the data is not compressed and can be used by the receiving device (display 20) without applying decompression. A controller 17 in device 10 implements control logic to route the uncompressed video content (UC) and/or compressed video content (C) to the interface part 11.

Fig. 2 shows another scenario in which the three devices 10, 20, 30 are daisy- chained together. Device 30 is a recording device which makes use of the compressed form of the AV content. As in Fig. 1, a device 10 outputs on interface part 11 content in both an uncompressed form and a compressed form. Device 30 receives the content, in both forms, via interface 31. The compressed form of the content is recorded on store 33 while the uncompressed form is passed through to output interface 32. Interface 32, 21 carries the uncompressed form of the content to display 20, where it can be displayed. It can be seen that by carrying the content in both an uncompressed form and a compressed form to recording device 30 means that the recording device does not need to compress the content before recording. Instead, the compressed form of the content can be stored directly on store 33. The output interface 32 can carry just the uncompressed form of the content (UC), or both the uncompressed and compressed forms of the content (UC + C). A controller 37 in device 30 implements control logic to route the compressed video content (C) from the interface part 31 to the store 33 (and output interface part 32). Fig. 3 shows another scenario in which the three devices 10, 20, 30 are daisy- chained together. Device 10 is positioned between recording device 30 and display 20. In this scenario, device sends a compressed form of the content across interface 16, 32 to recording device 30 and sends an uncompressed form of the content on interface 11, 21 to display 20. Optionally, device 10 can send both the uncompressed and compressed forms of the content across interface 11, 21. The compressed form of the content flows in an opposite direction to the uncompressed form of the content. In this embodiment, the compressed form of the content is carried without any accompanying uncompressed form of the content. Both forms of the content do not have to be carried together at all times. Fig. 4 shows another way in which the three devices 10, 20, 30 can be connected together. As described previously, an interface 11, 21 between device 10 and display 20 carries content in both an uncompressed form and a compressed form (UC + C). Display 20 uses the uncompressed form of the content for display. A further interface part 22 on display 20 receives the compressed form of the content. A further digital display interface 22, 32 between display 20 and recording device 30 carries the compressed form of the content to the recording device 30. As in Fig. 2, this means that the recording device 30 does not need to compress the content before recording. There are two notable features in this embodiment. Firstly, the compressed form of the video content is flowing away from display 20 (contrary to normal HDMI conventions). Secondly, the uncompressed and compressed forms of the content are sent to different destination devices: the uncompressed form of the content terminates at display 20 and the compressed for of the content terminates at recording device 30.

In a further scenario (not shown), an AV receiver is separately connected to a plurality of different AV source devices 10, each connection being made by a digital display interface, and a single output digital display interface connects the AV receiver to a display 20. The AV receiver includes a switching matrix to select one of the source devices, and connect the selected source to the display 20 via the output interface. Preferably, the switching matrix operates such that the second transport channel of each input and output port remain connected to each other i.e. it acts as a bus for the second transport channel. In this way, data can always be exchanged between two ports regardless of the state of the switching matrix.

A preferred Digital Display Interface (DDI) is the High-Definition Multimedia Interface (HDMI). HDMI defines three different connectors, called Type A, Type B and Type C. At present the most common connector is the Type A connector, as shown in Fig. 5. Each end of the connecting HDMI cable can be terminated in this manner and a socket on each of the interface parts 11, 16, 21, 22, 31, 32 is wired in the same manner. The Type A connector has 19 pins, each pin being connected to a respective one of the interface lines. A connecting cable is terminated at each end in a complementary manner. In HDMI versions 1.0-1.3, the pins of the type A connector are assigned as shown in Table 1 :

Conventional HDMI Pin Assignment, Type A connector (Table 1)

Pin 1 TMDS Data2+ Pin 2 TMDS Data2 Shield

Pin 3 TMDS Data2- Pin 4 TMDS Datal+ Pin 5 TMDS Datal Shield Pin 6 TMDS Datal-

Pin 7 TMDS DataO+ Pin 8 TMDS DataO Shield Pin 9 TMDS DataO- Pin 10 TMDS Clock+ Pin 11 TMDS Clock Shield Pin 12 TMDS Clock- Pin 13 CEC Pin 14 Reserved (N.C. on device) Pin 15 SCL Pin 16 SDA

Pin 17 DDC/CEC Ground Pin 18 +5 V Power Pin 19 Hot Plug Detect

Three Transition Minimized Differential Signaling (TMDS) channels (TMDS DataO, TMDS Datal, TMDS Data2) carry uncompressed video and audio data. Each TMDS channel uses three lines: a positive signaling line, and a negative signaling line and a shield. A TMDS clock uses a further three lines. Line 13 is used for a Consumer Electronics Control (CEC) Channel which is optional to implement, but mandatory to wire. This is a one-wire bidirectional bus which is generally used for AV Link remote control signaling. Line 14 is reserved and is not used. Lines 15 and 16 form an I²C bus (SCL = clock, SDA = data) which carries the Display Data Channel (DDC). The DDC allows a display to communicate it's specification to another device. Line 17 is a ground line shared by DDC and CEC. Line 18 carries +5 V power and line 19 is used to signal that the HDMI interface is ready and it can also be used to reset the HDMI interface. In a preferred embodiment of the invention, lines 13 and 14 of a Type A connector are not used in the way specified by the HDMI standard (at least as defined by HDMI 1.0-1.3) but are used as a bi-directional channel for carrying compressed AV content. The bi-directional channel is implemented as a Universal Serial Bus connection which carries the compressed AV content. USB Version 2.0 supports a data rate of 480Mbit/s, and is the preferred form of USB. However, less preferably, earlier versions of the USB specification could be used (USB Low Speed = 1.5Mbit/s; USB Full Speed = 12Mbit/s). The modified pin assignment of a Type A HDMI connector is shown in Table 2 (changes shown underlined):

Modified HDMI Pin Assignment, Type A connector (Table 2)

Pin 1 TMDS Data2+ Pin 2 TMDS Data2 Shield

Pin 3 TMDS Data2- Pin 4 TMDS Datal+

Pin 5 TMDS Datal Shield Pin 6 TMDS Datal-

Pin 7 TMDS DataO+ Pin 8 TMDS DataO Shield Pin 9 TMDS DataO- Pin 10 TMDS Clock+

Pin 11 TMDS Clock Shield Pin 12 TMDS Clock-

Pin 13 CEC/USB1 Pin 14 USB2

Pin 15 SCL Pin 16 SDA

Pin 17 DDC/CEC Ground Pin 18 +5 V Power Pin 19 Hot Plug Detect

Note that USBl and USB2 are the USB data lines D+ or D- (in either order). For an HDMI Type C connector, the CEC line (pin 14) and reserved line (pin 17) can be used to carry the USB connection. For an HDMI Type B connector, the CEC line (pin 22) and one of the reserved lines (e.g. pin 23) can be used to carry the USB connection. Fig. 6 shows an embodiment of the modified HDMI interface 50, 70 in more detail. Interface part 50 corresponds to parts 11, 32 shown in Figs. 1 to 3 and interface part 70 corresponds to parts 16, 21, 22, 31 shown in Figs. 1 to 3. The new hardware/functionality to implement a USB capable HDMI device is shown within boxes 60, 80. The additional functionality comprises a control 55, a pair of switches 56, 57, a CEC-USB bridge 58 and a USB transceiver 59. In use, the switches can be configured to connect directly to one another (position O', default position) or can be configured to connect to one another via the CEC- USB bridge 58 and USB transceiver 59 (position T).

At start-up, the normal DDC handshake is performed between devices and information is exchanged about the capabilities of the devices. When the USB control module 55 discovers that the other side of the interface is also USB capable, it sets the switches 56, 57 to position '1 '. Control 75 in interface part 70 sets also the switches 76, 77 to position '1 '. In this position, USB transceiver 59 connects via switches 57, 77 to USB transceiver 78. In this position, compressed AV data 53 is routed via USB transceiver 59 at a high bit rate to USB transceiver 78. The USB connection supports bi-directional transfer of compressed AV data, i.e. in the direction 50→70 and in the direction 70→50. While operating in this mode, conventional CEC commands are received on line 54 at switch 56 and routed to a CEC-USB bridge 58. The CEC commands on line 54 may have been generated by the device hosting interface part 50 or may have been generated by another device. The CEC- USB bridge 58 packages the CEC commands into a format which is compatible for transport over USB. Similarly, USB transceiver 59 examines data received via the USB connection. Any USB data packets that contain CEC data are routed to the CEC-USB bridge 58 and are translated into CEC protocol/message. In this way, CEC is bridged over USB. CEC-USB bridge 79 operates in the same manner. The bridging is achieved in such a way (electrically and logically) that a legacy CEC device cannot see the difference between an USB capable device and a legacy CEC device. Any device sending or receiving CEC commands which is connected to the CEC bus will be unaware that the bridging of CEC commands is occurring between interface parts 50, 70. When the USB control module 55 discovers that the other side of the interface is not USB capable, it leaves the switches 56, 57 in position '0'. This is the default position of the switches. This can occur when the other side of the interface is a legacy device which lacks the additional functional blocks 80. The information received from the EDID ROM 64 of the other interface part will lack any indication of the host device being USB capable. Similarly, if USB control module 75 in interface part 70 discovers that the other side of the interface is not USB capable, it leaves the switches 76, 77 in position '0'. In this position, the switches 56, 57 are connected directly to one another, by-passing the USB transceiver 59. Line 13 of the HDMI interface is used for CEC signaling in a manner which is entirely compatible with the conventional HDMI standard (table 1). In this position, compressed video or audio data 53 cannot be sent across the interface. When parts 50, 70 are turned off the CEC commands must be passed on transparently by the turned-off device. By leaving switches 56, 57 and 76, 77 in position O', this mimics the behavior required by the HDMI standard for handling CEC commands in power-off mode.

The embodiment described above uses the DDC channel to exchange information about the capabilities of interface parts 50, 70. Information about the capabilities of an interface part (i.e. whether it can support a USB connection) can be stored as part of the normal data in an Extended Display Identification Data (EDID) ROM. In an alternative embodiment, the CEC channel can be used to exchange information about which interface parts are USB capable. This can be particularly useful where more than two devices are daisy chained together and where it is necessary to establish the capabilities of an interface between a particular pair of devices (e.g. devices 10 and 30 in Figs. 2 and 3).

A USB connection requires a USB host-controller to be provided in at least one of the connected devices. A USB host-controller can form part of USB transceiver 78 in a display, as a display will usually be present in a chain of connected devices. The interface part in a device may support USB On-The-Go (OTG) in which each interface part has the capability to act as a host and connected devices negotiate to decide which device assumes the role of host. This allows greater flexibility in connecting devices together.

To ensure USB data can be reliably carried at high data rates, it is desirable that the lines of the HDMI cable carrying the USB data form a twisted pair. It is also necessary to ensure that the connector at each end of the cable has a connection made to the pins used by the USB channel. This is particularly relevant to any pins which are specified in the standard as "NC - not connected", such as pin 14 of a Type A HDMI connector. The interface can be used to carry both an uncompressed form and a compressed form of the same content, as shown in Figs. 1 to 4. The interface can also be used more flexibly. Fig. 7 shows two devices 20, 30 connected by an interface 22, 32 where the video data sent in uncompressed form UC(I) represents different content compared to the video data sent in compressed form C(2). The direction of transmission of the uncompressed and compressed forms of the content can be different, as shown in Fig. 7. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A digital display interface part for use in a first audio-visual device, the interface being for connecting the first audio-visual device to a second audio-visual device, the interface being defined by an industry standard and comprising a set of lines, the interface part comprising: - a first transport channel for carrying video data in uncompressed form; and, a second transport channel for carrying video data in compressed form, wherein the second transport channel uses a subset of lines of the interface which are reserved in the standard for a different use.

2. An interface part according to claim 1 wherein the second transport channel is capable of carrying audio and video data in compressed form.

3. An interface part according to claim 1 or 2 wherein the second transport channel supports bi-directional transport of compressed video data across the channel.

4. An interface part according to any one of the preceding claims further comprising control logic arranged to determine if the second device supports the second transport channel.

5. An interface part according to claim 4 wherein if the control logic determines that the second device does not support the second transport channel, the subset of lines are used in the manner defined in the industry standard.

6. An interface part according to any one of the preceding claims further comprising a Universal Serial Bus transceiver for supporting a Universal Serial Bus connection with the second device using the subset of lines of the interface, and wherein the second transport channel is carried over the Universal Serial Bus connection.

7. An interface part according to any one of the preceding claims wherein at least one of the subset of lines is reserved in the standard for carrying control data and wherein the control logic is further arranged to bridge the control data over the second transport channel.

8. An interface part according to claim 7 wherein the control data is Consumer

Electronics Control (CEC) data.

9. An interface part according to any one of the preceding claims wherein the second transport channel uses a line of the interface which is reserved in the standard as an unused line.

10. An interface part according to any one of the preceding claims wherein the digital display interface is an HDMI interface and the subset of lines are lines 13 and 14 of an HDMI Type A connector.

11. An audio-visual device comprising the interface part according to any one of the preceding claims.

12. An audio-visual device according to claim 11 further comprising: - an input for receiving compressed video content; a decoder for decompressing the compressed video content; and, control logic arranged to output the decompressed video content on the first transport channel and to output the compressed video content on the second transport channel.

13. An audio-visual device according to claim 12 further comprising control logic which is arranged to send or receive compressed video content on the second transport channel independently of sending or receiving uncompressed content on the first transport channel.

14. A method of distributing audio-visual data between a first audio-visual device and a second audio-visual device using a digital display interface defined by an industry standard, the interface comprising a set of lines, the interface comprising a first transport channel for carrying video data in uncompressed form, the method comprising, at the first audio -visual device: sending or receiving audio-visual data in a compressed form using a second transport channel of the interface, wherein the second transport channel uses a subset of lines of the interface which are reserved in the standard for a different use.

15. A method according to claim 14 further comprising sending or receiving, at the same time, video data in uncompressed form using the first transport channel of the interface.

16. A method according to claim 15 wherein the audio-visual data sent in compressed form represents different content compared to the video data sent or received in uncompressed form.

17. A method according to any one of claims 14 to 16 further comprising determining if the second device supports the second transport channel.

18. A method according to claim 17 wherein if it is determined that the second device does not support the second transport channel, the subset of lines are used in the manner defined in the industry standard.

19. Software comprising instructions for causing a processor to perform the method according to any one of claims 14 to 18.