Adaptive tuning of the perceptual model
View Patent ↗Methods of encoding a signal using a perceptual model are described in which a signal to mask ratio parameter within the perceptual model is tuned. The signal to mask ratio parameter is tuned based on a function of the bitrate of the part of the signal which has already been encoded and the target bitrate for the encoding process. The tuned signal to 5 mask ratio parameter is used to compute a masking threshold for the signal which is then used to quantise the signal.
1. A method of encoding a signal comprising:
inputting the signal to a perceptual model;
generating a masking threshold for the signal based on the signal and a signal to mask ratio parameter;
quantizing and encoding the signal based on the masking threshold; and
tuning the signal to mask ratio parameter, wherein tuning the signal to mask ratio parameter comprises:
calculating an average bitrate of the encoded portion; and
adjusting the signal to mask ratio parameters, wherein adjusting the signal to mask ratio parameter comprises:
SMR
(
adjusted
)
=
SMR
(
original
)
+
(
1
+
α
)
B
T
-
b
_
(
n
)
-
α
b
(
n
)
αβ
(
n
)
where B T is the target bitrate, b (n) is the average bitrate, b(n) is a short-term average bitrate calculated over a part of the encoded portion, β(n) is a tuning parameter and α is a predefined parameter.
2. The method according to claim 1 , further comprising:
repeating tuning the signal to mask ratio parameter periodically.
3. The method according to claim 2 , wherein the signal is divided into a sequence of frames and repeating tuning the signal to mask ratio parameter periodically comprises:
repeating tuning the signal to mask ratio parameter every N frames, where N is an integer.
4. The method according to claim 1 , wherein the adjustment of the signal to mask ratio parameter is further based on a function of a short-term average bitrate calculated over a part of the encoded portion.
5. The method according to claim 4 , wherein the part of the encoded portion comprises N frames, where N is an integer.
6. The method according to claim 1 , wherein the adjustment of the signal to mask ratio parameter is further based on a tuning factor.
7. The method according to claim 6 , further comprising:
updating the tuning factor based on a measured change in bitrate.
8. The method according to claim 7 , wherein the tuning factor is updated using:
β
(
n
+
1
)
=
M
β
(
n
)
+
Δ
b
(
n
)
Δ
SMR
M
+
1
where ΔSMR is a previous change in signal to mask ratio parameter, Δb(n) is a corresponding resultant change in the short-term average bitrate and M is a smoothing factor.
9. The method according to claim 1 , wherein tuning the signal to mask ratio parameter based on at least a function of a bitrate of an encoded portion of the signal and a target bitrate further comprises:
limiting any change in signal to mask ratio parameter.
10. The method according to claim 1 , wherein the perceptual model comprises a psychoacoustic model and the signal comprises an audio signal.
11. An encoder comprising:
a perceptual model arranged to generate a masking threshold far a signal based on the signal and a signal to mask ratio parameter;
a bit allocation and quantization element arranged to quantize the signal based on the masking threshold;
a bitstream formatting element arranged to encode the signal based on the masking threshold; and
a tuning element arranged to:
calculate an average bitrate of the encoded portion: and
adjust the signal to mask ratio parameter by computing:
SMR
(
adjusted
)
=
SMR
(
original
)
+
(
1
+
α
)
B
T
-
b
_
(
n
)
-
α
b
(
n
)
αβ
(
n
)
where B t is the target bitrate, b (n) is the average bitrate, b(n) is a short-term average bitrate calculated over a part of the encoded portion, β(n) is a tuning parameter and α is a predefined parameter.
12. The encoder according to claim 11 , wherein the adjustment of the signal to mask ratio parameter is further based on a function of a short-term average bitrate calculated over a part of the encoded portion.
13. The encoder according to claim 11 , wherein the adjustment of the signal. to mask ratio parameter is further based on a tuning factor.
14. The encoder according to claim 11 , wherein the tuning element is further arranged to:
limit any change in signal to mask ratio parameter.
15. The encoder according to claim 11 , wherein the perceptual model comprises a psychoacoustic model and the signal comprises an audio signal.
16. A method of encoding a signal comprising:
inputting the signal to a perceptual model;
generating a masking threshold for the signal based on the signal and a signal to mask ratio parameter;
quantizing and encoding the signal based on the masking threshold; and
tuning the signal to mask ratio parameter, wherein tuning the signal to mask ratio parameter comprises:
calculating an average bitrate of the encoded portion;
adjusting the signal to mask ratio parameter based on at least a function of an average bitrate and a target bitrate for the signal and a tuning factor; and
updating the tuning factor using:
β
(
n
+
1
)
=
M
β
(
n
)
+
Δ
b
(
n
)
Δ
S
M
R
M
+
1
where ΔSMR is a previous change in signal to mask ratio parameter, Δb(n) is a corresponding resultant change in a short-term average bitrate and M is a smoothing factor.
17. An encoder comprising:
a perceptual model arranged to generate a masking threshold for a signal based on the signal and a signal to mask ratio parameter;
a bit allocation and quantization element arranged to quantize the signal based on the masking threshold;
a bitstream formatting element arranged to encode the signal based on the masking threshold; and
a tuning element arranged to calculate an average bitrate of the encoded portion, adjust the signal to mask ratio parameter based on at least a function of the average bitrate and the target bitrate for the signal and a tuning factor, and update the tuning factor using:
β
(
n
+
1
)
=
M
β
(
n
)
+
Δ
b
(
n
)
Δ
S
M
R
M
+
1
where ΔSMR is a previous change in signal to mask ratio parameter, Δb(n) is a corresponding resultant change in a short-term average bitrate and M is a smoothing factor.