GB2235599A

GB2235599A - Analogue to digital conversion systems

Info

Publication number: GB2235599A
Application number: GB9014439A
Authority: GB
Inventors: John Robert Emmett
Original assignee: Thames Television PLC
Current assignee: Thames Television PLC
Priority date: 1989-07-01
Filing date: 1990-06-28
Publication date: 1991-03-06
Anticipated expiration: 2010-06-28
Also published as: GB2235599B; GB9014439D0

Abstract

An analogue to digital conversion system superimposes (5) a dither signal (9) on a filtered (1) analogue input signal 19, the amplitude of the dither signal being varied (3, 7) in accordance with the amplitude of the input signal. After conversion (11) the element of the digital signal deriving from the dither signal is removed (15). The system allows for improved audio coding. <IMAGE>

Description

ANALOGUE TO DIGITAL CONVERSION SYSTEMS This invention relates to analogue to digital conversion systems. The invention has particular, although not exclusive relevance to analogue to digital conversion systems for use in digital audio coding.

The need for ever higher resolution in digital audio coding puts increasing demands on analogue to digital conversion systems.

In U.S. Patent No. 4,710,747 there is disclosed an analogue to digital conversion system in which the accuracy and resolution of the analogue to digital conversion is increased by adding a high frequency "dither" to the analogue signal to ensure that the analogue to digital transfer conversion is performed over the linear part of the analogue to digital characteristic.

Such a system is not, however, appropriate for use in digital audio coding as for high audio signal levels the "dither" signal will overload the analogue to digital conversion system.

It is an object of the present invention to provide an analogue to digital conversion system suitable for use in digital audio coding.

According to the present invention, an analogue to digital conversion system for an input analogue signal comprises means for superimposing a dither signal of higher frequencies than the input signal on the input signal, the amplitude of the dither signal varying in accordance with the amplitude of the instantaneous input signal; means for deriving a digital signal representative of the input signal together with the superimposed dither signal; and means for filtering out the component of the digital dither signal in the digital signal.

Preferably the system comprises an attenuator for controlling the amplitude of the dither signal and a comparator for responding to the amplitude of a portion of the input analogue signal and for providing to the attenuator a control signal dependent upon this amplitude.

Preferably the system further comprises a digital to analogue converter for receiving the digital signal and a further comparator means for comparing an analogue output signal from the digital to analogue converter with a part of the input signal thereby to provide a frequency component of the dither signal.

Two particular analogue to digital conversion systems in accordance rith the invention will now be described, by way of example only, with reference to the accompanying figures in which: Figure 1 is a schematic diagram of the system; Figure 2 illustrates the transfer characteristic of an ideal analogue to digital conversion system; Figure 3 illustrates the transfer characteristics of an analogue to digital conversion system with varying amounts of dither; and Figure 4 is a schematic diagram of an alternative system in accordance with the invention.

Referring firstly to Figure 1, the system is designed for use in digital audio coding and includes a low pass filter 1 having its upper passband limit set at the system Nyquist frequency fl.

The output of the low pass filter 1 is divided between a comparator 3 and an input of a summing circuit 5. The output of the comparator 3 is connected to a control attenuator 7 to which the output of a dither generator 9 is also connected. The output of the control attenuator is connected to a further input of the summing circuit. The output of the summing circuit 5 is connected to an analogue to digital conversion circuit 11 including a sampling circuit clocked by sampling clock 13. The digital output of circuit 11 is connected via a further low pass filter 15, having its upper passband limit set at frequency fl, to a digital output port 17.

In use of the system an analogue audio input signal is applied to the input terminal 19 of the low pass filter 1. The comparator 3 responds to the amplitude of the filtered analogue audio input signal afforded thereto to produce a signal to the attenuator 7 dependent on this amplitude. The dither generator 9 produces a signal with frequency components between fl and nfl where n is the oversampling ratio of the conversion system as will be explained hereafter. The attenuator 8 is effective to attenuate the dither signal produced by the generator 9 to an extent determined by the amplitude of the measured portion of filtered analogue audio input signal.

Thus the input signal to the sampling and analogue to digital converter circuit 11 is a signal with frequency components up to fl, with a superimposed dither signal having components ranging from fl to nfl. The effect of the dither signal is to add a high level of statistical noise to the signal. The clock 13 produces clock pulses at a rate of 2n times fl. Thus the filtered analogue signal, with superimposed dither signal, is oversampled at 2n times the Nyquist frequency for the audio signal. The low pass filter 15 removes the dither signal component to re-establish the fl upper frequency limit in the digital output signal.

It will be appreciated that this oversampling technique is effective to produce a statistically more accurate digital signal. By increasing the sampling rate to a value above twice the Nyquist frequency 2fl the quantisation noise is effectively "smeared" over correspondingly increased frequency range and the noise power in the filtered output signal is correspondingly reduced.

The operation of the system is further illustrated by reference to Figures 2 and 3. Curve A in Figure 3 shows the typical transfer characteristic of an analogue to digital conversion system using oversampling, but employing no dither signal. This curve may be compared with curve B, in which a low amplitude dither signal is superimposed on an audio signal. For low signal levels, where the analogue to digital conversion errors are often the largest, there is still a high level of amplitude non-linearity present in the transfer characteristic. For large signal amplitudes however the characteristic exhibits adequate linearity.Referring to curve C, which shows the equivalent transfer characteristic for an audio signal with a superimposed dither signal of relatively high amplitude, the transfer characteristic linearity for small signal amplitudes has good linearity, although the high signal level transfer characteristic linearity is poor due to the asymptotic approach to the overload characteristic of the analogue to digital conversion circuit 11.

Thus by variation of the amplitude of the dither signal according to the instantaneous amplitude of the input signal, an optimum linear transfer characteristic can be obtained regardless of individual analogue to digital conversion errors, i.e. a transfer characteristic approaching that shown in Figure 2.

Referring now to Figure 4, the second analogue to digital conversion system to be described is an adaption of the first system, and thus like components are accordingly labelled. The second system incorporates however a feedback loop from the output of the analogue to digital conversion circuit 11 to the attenuator 7. Part of the digital signal produced by the circuit 11 is converted back into an analogue signal by digital to analogue circuit 19, and compared with part of the input audio signal by comparator 21. The output of the comparator 21 is filtered by filter 23 to provide a further input to the attenuator 7. This signal forms a further component of the dither signal added to the filtered audio signal, and acts to create further linearity in the transfer characteristic of the system.

Claims

1. An analogue to digital conversion system for an input analogue signal comprises; means for superimposing a dither signal of higher frequencies than the input signal on the input signal, the amplitude of the dither signal varying in accordance with the amplitude of the instantaneous input signal; digitiser means for deriving a digital signal representative of the input signal together with the superimposed dither signal; and first filter means for filtering out the component of the digital dither signal in the digital signal.

2. A system according to claim 1 comprising an attenuator for controlling the amplitude of the dither signal and a comparator for responding to the amplitude of a portion of the input analogue signal and for providing to the attenuator a control signal dependent upon this amplitude.

3. A system according to claim 1 or claim 2 wherein the digitiser means is arranged to oversample the analogue signal with superimposed dither signal by factor n, the dither signal having frequency components in the range fl to nfl, where fl is system Nyquist frequency.

4. A system according to claim 3 wherein the digitiser means is arranged to sample the analogue signal with superimposed dither signal at twice the highest frequency component of the dither signal.

5 A system according to any one of the preceding claims comprising further filter means for filtering the input analogue signal.

6 A system according to claim 5 wherein the first and further filter means are arranged to operate at system Nyquist frequency fl.

7. A system according to any one of the preceding claims further comprising a digital to analogue converter for receiving the digital signal and a further comparator means for comparing an analogue output signal from the digital to analogue converter with a part of the input signal thereby to provide a frequency component of the dither signal.

8. A system according to claim 7 comprising additional filter means for filtering the output signal from the further comparator means.

9. A system as substantially hereinbefore described with reference to the accompanying drawings.