Frequency band extension in an audio signal decoder
A method is provided for extending the frequency band of an audio signal during a decoding or improvement process. The method includes obtaining the decoded signal in a first frequency band, referred to as a low band. Tonal components and a surround signal are extracted from the signal from the low-band signal, and the tonal components and the surround signal are combined by adaptive mixing using energy-level control factors to obtain an audio signal, referred to as a combined signal. The low-band decoded signal before the extraction step or the combined signal after the combination step are extended over at least one second frequency band which is higher than the first frequency band. Also proved are a frequency-band extension device which implements the described method and a decoder including a device of this type.
1. A method for extending a frequency band of an audio frequency signal the signal having been decoded in a first frequency band termed the low band during a decoding or improvement process, the method comprising:
obtaining an excitation signal from said audio frequency signal in a first frequency band;
extracting tonal components from the excitation signal resulting in a residual excitation signal comprising detecting and deleting dominant tonal components of the excitation signal by estimating the frequency and amplitude parameters, wherein estimation of the frequency parameters is performed by parabolic interpolation;
extracting an ambience signal from the residual excitation signal;
combining the extracted tonal components and the extracted ambience signal by adaptive mixing using energy level control factors to obtain an audio signal, termed a combined signal, a control factor for the energy level used for the adaptive mixing being computed as a function of the total energy of the excitation signal or excitation and extended signal and of the tonal components; and
extending in at least one second frequency band the excitation signal before the extraction of the tonal components or before the tonal components are combined with the ambience signal, and
wherein the act of extending the excitation signal is performed according to the following equation:
U
HB
1
(
k
)
=
{
0
k
=
0
,
…
,
199
U
(
k
)
k
=
200
,
…
,
239
U
(
k
+
start_band
-
240
)
k
=
240
,
…
,
319
with k the index of the sample, U(k) the spectrum of the decoded low band signal obtained after a transform act, U HB1 (k) the spectrum of the extended signal, and start_band a predefined variable.
2. The method as claimed in claim 1 , wherein the extracting of the tonal components and of the ambience signal is performed according to the following acts:
detecting the dominant tonal components of the decoded or decoded and extended low band signal, in the frequency domain;
computing a residual signal by extraction of the dominant tonal components to obtain the ambience signal.
3. The method as claimed in claim 1 , wherein the extracting of the tonal components and of the ambience signal is performed according to the following acts:
obtaining the ambience signal by computing a mean value of the spectrum of the excitation signal;
obtaining the tonal components by subtracting the ambience signal from the excitation signal or the extended excitation signal.
4. The method as claimed in claim 1 , wherein the decoded low band signal undergoes an act of transform or filter bank-based sub-band decomposition, the extracting and combining acts then being performed in the frequency or sub-band domain.
5. The method as claimed in claim 1 , wherein the decoded low band signal is a low band decoded excitation signal.
6. A device for extending frequency band of an audio frequency signal the signal having been decoded in a first frequency band termed the low band, the device comprising:
a computer-readable memory that is not a transitory propagating wave or signal, and comprising instructions stored thereon;
a processor configured by the instructions to perform acts comprising:
obtaining an excitation signal from said audio frequency signal in said first frequency band;
extracting tonal components from the excitation signal resulting in a residual excitation signal comprising detecting and deleting dominant tonal components of the excitation signal by estimating the frequency and amplitude parameters, wherein estimation of the frequency parameters is performed by parabolic interpolation;
extracting an ambience signal from the residual excitation signal;
combining the extracted tonal components and the extracted ambience signal by adaptive mixing using energy level control factors to obtain an audio signal, termed a combined signal, a control factor for the energy level used for the adaptive mixing being computed as a function of the total energy of the excitation signal or excitation and extended signal and of the tonal components; and
extending in at least one second frequency band the excitation signal before the extraction of the tonal components or before the tonal components are combined with the ambience signal, and
wherein the act of extending the excitation signal is performed according to the following equation:
U
HB
1
(
k
)
=
{
0
k
=
0
,
…
,
199
U
(
k
)
k
=
200
,
…
,
239
U
(
k
+
start_band
-
240
)
k
=
240
,
…
,
319
with k the index of the sample, U(k) the spectrum of the decoded low band signal obtained after a transform act, U HB1 (k) the spectrum of the extended signal, and start_band a predefined variable.
7. A computer-readable storage medium that is not a transitory propagating wave or signal, and on which is stored a computer program comprising code instructions for execution of a frequency band extension method when executed by a processor of a frequency band extension device, the instructions configuring the frequency band extension device to perform acts comprising:
obtaining an excitation signal from an audio frequency signal, the audio frequency signal having been decoded in a first frequency band termed the low band;
extracting tonal components from the excitation signal resulting in a residual excitation signal comprising detecting and deleting dominant tonal components of the excitation signal by estimating the frequency and amplitude parameters, wherein estimation of the frequency parameters is performed by parabolic interpolation;
extracting an ambience signal from the residual excitation signal;
combining the extracted tonal components and the extracted ambience signal by adaptive mixing using energy level control factors to obtain an audio signal, termed a combined signal, a control factor for the energy level used for the adaptive mixing being computed as a function of the total energy of the excitation signal or excitation and extended signal and of the tonal components; and
extending in at least one second frequency band the excitation signal before the extraction of the tonal components or before the tonal components are combined with the ambience signal, and
wherein the act of extending the excitation signal is performed according to the following equation:
U
HB
1
(
k
)
=
{
0
k
=
0
,
…
,
199
U
(
k
)
k
=
200
,
…
,
239
U
(
k
+
start_band
-
240
)
k
=
240
,
…
,
319
with k the index of the sample, U(k) the spectrum of the decoded low band signal obtained after a transform act, U HB1 (k) the spectrum of the extended signal, and start_band a predefined variable.