MXPA00003666A - Background noise contrast reduction for handovers involving a change of speech codec - Google Patents

Abstract

A method and apparatus for smooth transitions of background noise as a mobile communication unit roams from a first communication system having a first speech encoder (310) to a second communication system having a second speech encoder (330). As the mobile communication unit roams into the second communication system, the first communication system will transfer the communication signal to the second communication system. Background noise from the first communication system is sampled (320), and as the communication signal is transferred, the noise signal sampled in the first communication system is mixed in a mixer of the second communication system with background noise sampled (340) from the second communication system. As the change in noise progresses, the noise from the first communication system will gradually be decreased and faded out while the noise from the second communication system will be gradually increased to a high power level.

Description

REDUCTION OF BACKGROUND NOISE CONTRAST FOR DELIVERIES INVOLVING A CHANGE OF VOCAL FREQUENCY ENCODER / DECODER Field of the Invention The present invention relates generally to the quality of speech frequency in a communication system, and in particular to the reduction of noise contrast in a voice frequency codee (encoder / decoder) delivery.

BACKGROUND OF THE INVENTION Many voice frequency codes provide the ability for the vocal frequency to be transported with a long-distance transmission quality. However, different codeine technologies (encoder / decoder) behave differently in the encoding of signals that are not of vocal frequency, and in particular, background noise. For example, ADPCM at 32 kb / s (kilobits per second), which has spread, has developed the use of a voice-frequency codec for short-range wireless and digital systems (such as the CT2, DECT, PHS and PACS), offers an almost transparent quality for any background noise. The Linear Prediction Analysis by Synthesis (LPAS) codes (such as the CELP, VSELP and ACELP systems) that are predominantly used in cellular and digital PMR systems (such as GSM, IS-54, IS-95, IDEN, and TETRA) encode background noise with a completely different character Mobile systems generally have background noise in the transmissions, although noise reduction is possible. Mobile systems include background noise because it is well known that most users prefer that some low level of acceptable noise be present to indicate that the call is still active. Some systems, such as the UMTS (Universal Mobile Telephony System), allow deliveries between wireless / short-range environments and traditional cellular environments and from cellular to wireless / short-range environments. The UMTS system is designed to allow a user to move around the world with a single mobile phone unit and be able to communicate with any system in that area. The UMTS system allows linking different systems in a single area, such as wireless and cellular. In this type of system, a user can start a call on his wireless system and then start moving out of range of that system. To keep the call, the call is delivered from one system to another, such as from a wireless system to a cellular system. The call also moves from the cellular system to a wireless system when the user moves into the range of the wireless area. As mentioned above, each voice frequency coder or codec has somewhat different design parameters and requirements. Because of this, a user will hear different background noises when using different communication systems that incorporate different codees. Certainly during a delivery, where two different systems are incorporated that have different design parameters, the user will be exposed to different codes and therefore different background noises. In a system where more than one codee is used, such as the UMTS system mentioned above, it is desirable that users under delivery conditions are relatively incapable of making changes between the characteristics of the background noise. UMTS mainly provides seamless transfers, and although seamless usually means without interruption, obvious background noise contrasts will occur during deliveries between systems that exploit different codeine technologies.

Brief Description of the Drawings Figure 1 is a diagram of a mobile unit in a delivery condition. Figure 2 is a diagram of a speech frequency signal during a delivery according to a preferred embodiment of the present invention. Figure 3 is a diagram of the elements used in the preferred embodiment of the present invention. Figure 4 is a flow diagram of the method used by the preferred embodiment of the present invention.

Detailed Description of a Preferred Modality The present invention provides a method and apparatus for enabling uniform transitions of background noise when a mobile communication unit 110 (of Figure 1) wanders from a first communication system 120 having a first encoder ( codee) of speech frequency to a second communication system 130 having a second voice frequency coder. When the mobile communication unit 110 wanders to the second communication system 130, the first communication system 120 will transfer the communication signal to the second communication system 130, transfer which is called delivery. A speech activity detection system which is well known in the art is applied to the vocal frequency to discriminate between the moments where vocal frequency and background noise are present and the moments when only background noise is present. During the periods with background noise only, the background noise of the first communication system 120 is sampled, and when the communication signal is transferred, the noise signal sampled in the first communication system 120 is mixed in a mixer of the second communication system 130 with the background noise sampled from the second codec. Initially the noise of the first communication system 120 will have a high relative power level and the noise of the second communication system 130 will have a low relative power level. As the delivery progresses, the noise of the first communication system 120 (initial noise) will gradually decrease and disappear while the noise of the second communication system 130 will gradually increase to a higher relative power level. The result of the gradual transition of the noise of the first communication system 120 to the noise of the second communication system 130 is such that the user of the mobile communication unit can not notice a change in the noise level from one system to another.

Referring now to Figure 2, signal patterns 202 and 204 of a first codee and a second codee, respectively, are shown. The "Cl" element (206) of the signal pattern 202 indicates segments of active voice frequency codee by codee 1. The element "C2" (208) of the signal pattern 204 indicates segments of the vocal frequency codee by codee 2. The element " NI "(210) of the vocal frequency segment 202 represents periods of" silence "between the vocal frequency segments 206 and the element" N2"(212) of the vocal frequency segment represents periods of" silence "between the vocal frequency segments 208. At the point of delivery (214), where the communication signal is transferred from code 1 to code 2, code 2 will have a different noise level than Ni (210). If the noise of the codec 2 were immediately inserted in the periods of silence N2 (212), a user would probably not notice the difference in the background noise. Accordingly, in the preferred embodiment of the present invention shown in Figure 2, the sampled background noise of the codec 1 is inserted in any initial silence period (216) that occurs near the delivery point. For example, the period of silence 216 is simply NI. Since the change in noise Ni to the N2 continues to progress, the noise N2 of the codec 2 has been sampled during the silence period 216 and is mixed with the noise NI to form a combined background noise level in the silence period 218. Initially the noise level N2 is low while the noise NI is high. In the period of silence 220, the noise N2 decreases further while the noise NI decreases proportionally. The noise N2 in the silence period 222 increases further and NI decreases further. With each period of silence (218 to 224), the noise level of the two sounds NI and N2 sampled change with the decrease of NI while N2 increases to a point where only the noise N2 remains. Figure 3 shows a diagram of the elements used in the present invention. Specifically, a first tranceptor having a codee 310 is transmitting the initial communication signal with the initial background noise. During delivery, the noise sampler 320 samples the background noise of the communication signal received from codex 310. At the same time during delivery, a second transceiver having a codex 330 is communicating with the mobile unit. The background noise of the codec 330 is sampled in the noise sampler 340. The background noise of both the noise sampler 320 and the sampler 340 are each supplied with a given amount of gain Gl and G2 respectively in the multipliers 350 and 360 respectively. The two noise streams are then combined in the adder 370 to give the combined noise which is received by the transceiver of the mobile unit 380. To gradually lower the background noise of the noise sampler 320 and increase the background noise of the noise sampler 340, the gain for Gl and for G2 is determined in the following manner. The gains Gl and G2 are calculated at time "t" of the power interpolation function r (t). It should be noted that the power interpolation function is well known in the art and will not be discussed in detail. The power interpolation function can be done linearly or by any appropriate monatomic function. Assuming a nomenclature of pl for the background noise power of codee 1 and p2 for the background noise power of codee 2, the power of the combined background noise (pT) at time t is: pT = pl * r (t ) + p2 (1 - r (t)) The initial estimates of Gl and G2 are then given by: Gl = Square Root of (r (t)) G2 = Square Root of (1 - r (t)) The gains Gl and G2 are then applied to the two noise sources as shown in FIGURE 3. The total noise power is calculated and Gl and G2 are adjusted by means of a common multiplier to ensure that the total output power is equal to pT. The change in the power interpolation function through the noise change adjusts the values of Gl and G2 causing Gl to gradually shift to zero while G2 gradually increases. Through the change with Gl and the adjustment of G2 according to the change of the power interpolation function, the value of pT must remain constant. FIGURE 4 describes the process followed by the preferred embodiment of the present invention. The background noise is sampled from each codee 1 and codee 2 (410). The background noise of the codec 1 is replaced in the first silence period of the codec 2 (216) (in step 420). The initial gain for code 1 noise during delivery is estimated according to equation 2 above (430). The initial gain for codec 2 noise in the delivery is estimated according to equation 3 above (440) The noise of each one is multiplied by their respective gains (450 and 460) and the adjusted noise levels are mixed (470). As mentioned above, the total output power must be equal to pT calculated in the transfer. If the noise change is not finished, a new r (t) (490) is calculated and new gain values are determined (430 and 440). The process of adjusting the background noise levels for codee 1 and codee 2 by adjusting the gain continues until the change is complete (all the noise NI has been put out of phase leaving only the noise N2) at which time it ends the process (495). Using the method and apparatus taught in the preferred embodiment of the present invention, a uniform background noise transition is provided from a first communication system using a first voice frequency coder to a second communication system utilizing a speech coder. different vocal frequency. The gradual change from one background noise to another means that the user will not be able to notice the delivery due to the different noise levels. The present invention can be used in any system where different voice frequency coders are incorporated. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (10)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A method, characterized in that it comprises the steps: sampling a period of noise for each of a first and second voice frequency coders, where a communication from the first voice frequency coder to the second voice frequency coder is transferred; and mixing the mixed noise period of the first and second voice frequency coders for the transfer of communication. The method according to claim 1, characterized in that the step of mixing the sampled period comprises mixing the sampled noise period of the second voice frequency coder from an initial low relative power level to a higher relative power level. The method according to claim 2, characterized in that the step of mixing comprises estimating a gain of the sampled noise period of the first speech frequency coder according to the square root of the equation of a power interpolation function. The method according to claim 3, characterized in that the mixing step comprises estimating a gain of the sampled noise period of the second speech frequency coder according to the square root of the equation of one minus the power interpolation function . The method according to claim 1, characterized in that the step of mixing the sampled period comprises sampling the sampled noise period of the first speech frequency coder from an initial high power level to a power level of zero. 6. A communication system, characterized in that it comprises: a first and a second voice frequency coder, each of the voice frequency coders has different background noise; and a mixer that receives samples of the background noise from the first voice frequency coder and mixes background noise from the first voice frequency coder with the background noise from the second voice frequency coder to transfer a communication between the first and second frequency coders vocal. The communication system according to claim 6, characterized in that the mixer mixes the background noise of the first and second voice frequency coders in different degrees to gradually increase a power level of the background noise of the second voice frequency coder. and gradually lowering the background noise of the first voice frequency coder. 8. A transceiver, characterized in that it comprises: a noise sampler which samples noise in a signal transmission of a first transceiver voice frequency encoder; and a mixer that receives the noise from the noise sampler and also receives the sampled noise from a signal transmission from a second voice frequency coder, the mixer combines the noise from the first and second voice frequency coders for a transmission delivery of signals from one of the first and second voice frequency coders to one of the first and second voice frequency coders. The transceiver according to claim 8, characterized in that the mixer comprises means for gradually fading an initial sampled noise from the first and second voice frequency coders. 10. A transceiver that wanders between a first and a second communication system having different voice frequency encoders, characterized in that the transceiver obtains from the second communication system a mixed noise signal having sampled noise from each of the different encoders of vocal frequency during the delivery of a signal from the first to the second communication system.