Method and apparatus for estimating a parameter for low bit rate stereo transmission

1. A method for estimating panning gain parameters for low bit rate stereo transmission, comprising the steps of:

a. deriving estimate of time delay between left and right audio channels in a multi-channel signal from a time delay subsystem, wherein the time delay system employs an inter-channel time difference (ITD) processor for;

i. receiving the left audio signal from a first microphone and receiving the right audio signal from a second microphone;

ii. downsampling the left and right audio signals to a lower bandwidth and sampling rate;

iii. producing a windowed and normalized cross correlated signal of the left and right audio signals;

b. employing cross-correlation between the left and right audio channels in the time delay subsystem;

c. employing a normalized cross-correlation within an inter-channel intensity difference (IID) processor; and

d. deriving an estimate of panning gains for the left and right audio channels from the IID processor.

2. The method claimed in claim 1 , further comprising the step of coupling an encoded mono stereo signal with bits that represent panning gains corresponding to the left and right audio channels such that a low-bit rate parametric stereo signal is produced.

3. A method for switching a stereo encoding technique from a high bit rate full stereo technique to a low bit rate parametric technique wherein the cause of the switching corresponds to either bit-rate constraint or bit-rate relaxation and wherein the method comprises the steps of:

a. determining whether bit-rate constraint or bit-rate relaxation is employed;

b. providing the low bit rate parametric stereo signal in a manner that comprises:

(1) operating independently upon the left and right audio signals to yield independent panning gains corresponding to left and right audio signals using a combination of a cross-correlation of left and right audio channels, a linear predictive coefficient (LPC) gain independently calculated in a decimated domain for the left and right audio signals, and energy values corresponding to the left and right audio signals; and

(2) coupling with an encoded mono signal to produce the low bit rate parametric signal; and alternatively

c. providing the high bit rate full stereo signal in a manner that comprises:

(1) receiving a left and right audio channel from a multi-channel signal

(2) determining an inter-channel time difference between the left and right audio channels;

(3) compensating both left and right channels according to the inter-channel time difference; and

(4) encoding the left and right audio channels either jointly or independently to produce a higher quality stereo signal representation comprising a stereo signal that has increase in bit rate by at least 25% when compared to an equivalent mono signal.

4. An apparatus with functionality to encode a stereo signal at either a high-bit rate or a low-bit rate with encoding selection that is dependent upon either a signal source or bandwidth constraint, the encoder comprising:

a parametric processor operable upon both a left and right audio signal, wherein the parametric processor yields independent panning gains corresponding to the left and right audio signals wherein a panning gain corresponding to the left audio signal (g left ) is found using:

g

left

=

2.0

1

+

C

⁢

⁢

C

⁢

⁢

F

⁡

(

G

L

⁢

G

R

)

E

L

⁢

G

L

2

where CCF is a cross-correlation of left and right audio channels, G L is a linear predictive coefficient (LPC) gain calculated in a decimated domain for the left audio signal, and E L is value of left audio signal energy; and

wherein a panning gain corresponding to the right audio signal (g right ) is found using:

g

right

=

2.0

1

+

C

⁢

⁢

C

⁢

⁢

F

⁡

(

G

L

⁢

G

R

)

E

R

⁢

G

R

2

where CCF is a cross-correlation of left and right audio channels, linear predictive coefficient (LPC) gain calculated in a decimated domain for the right audio signal, and E R is value of right audio signal energy.

5. The apparatus claimed in claim 4 , wherein the panning gains are calculated using frequency components below 2 kHz.

6. The apparatus claimed in claim 4 , wherein the panning gains are calculated from a peak cross-correlation in a decimated linear predictive coefficient (LPC) residual domain of the first and second audio signals.

7. The apparatus claimed in claim 4 , wherein the panning gains are encoded and transmitted with a single bit per a speech frame.

8. The apparatus claimed in claim 4 , wherein the first and second audio signals are stereo speech or voice signals.

9. The apparatus claimed in claim 8 , wherein the stereo speech or voice signals are transmitted at 100-400 bits per second (bps) along with transmission of mono speech signals.

10. An apparatus that encodes a stereo signal at a high-bit rate and a low-bit rate with selection that is dependent upon either a signal source or bandwidth constraint, the apparatus comprising:

a. a microphone system providing a first audio signal and a second audio signal wherein the second audio signal has a time difference from the first audio signal;

an analyzer coupled to the microphone system that determines an inter-channel time difference between the first audio signal and the second audio signal, by employing an inter-channel time difference (ITD) processor for;

i. receiving the left audio signal from a first microphone and receiving the right audio signal from a second microphone;

ii. downsampling the left and right audio signals to a lower bandwidth and sampling rate;

iii. producing a windowed and normalized cross correlated signal of the left and right audio signals and;

b. a parametric processor coupled to the analyzer that calculates panning gains of the first and second audio signals on a frame-by-frame basis; and

c. an encoder coupled to the processor so that an encoded mono signal is coupled with the panning gains of the first and second audio signals and the inter-time difference signal corresponding to the first and second audio signals.

11. A computer-readable storage medium having computer readable code stored thereon for programming a computer to perform a method of estimating panning gain parameters for low bit rate stereo transmission, comprising the steps of:

a. deriving estimate of time delay between left and right audio channels in a multi-channel signal from a time delay subsystem, wherein the time delay system employs an inter-channel time difference (ITD) processor for;

i. receiving the left audio signal from a first microphone and receiving the right audio signal from a second microphone;

ii. downsampling the left and right audio signals to a lower bandwidth and sampling rate;

iii. producing a windowed and normalized cross correlated signal of the left and right audio signals;

b. employing cross-correlation between the left and right audio channels in the time delay subsystem;

c. employing a normalized cross-correlation within an inter-channel intensity difference (IID) processor; and

d. deriving an estimate of panning gains for the left and right audio channels from the IID processor.