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Add stub for Limiting the Prediction Gain of the LPC Filter

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Comes from 4.2.7.5.8 we can implement this later
This commit is contained in:
Sean DuBois 2022-09-05 15:41:26 -04:00
parent 1e8a69eba3
commit 5919d29d78
2 changed files with 62 additions and 16 deletions
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77
internal/silk/decoder.go
View file

@ -12,6 +12,9 @@ type Decoder struct {
// Have we decoded a frame yet?
haveDecoded bool
// Is the previous frame a voiced frame?
isPreviousFrameVoiced bool
// TODO, should have dedicated frame state
logGain uint32
subframeState [4]struct {
@ -385,7 +388,7 @@ func (d *Decoder) normalizeLineSpectralFrequencyCoefficients(bandwidth Bandwidth
// percentile of a large training set).
//
// https://datatracker.ietf.org/doc/html/rfc6716#section-4.2.7.5.4
func (d *Decoder) normalizeLSFStabilization() {
func (d *Decoder) normalizeLSFStabilization(nlsfQ15 []int16) {
// TODO
}
@ -395,7 +398,7 @@ func (d *Decoder) normalizeLSFStabilization() {
// (in the same channel) and the current frame
//
// https://datatracker.ietf.org/doc/html/rfc6716#section-4.2.7.5.5
func (d *Decoder) normalizeLSFInterpolation() error {
func (d *Decoder) normalizeLSFInterpolation(nlsfQ15 []int16) (n1Q15 []int16, err error) {
// Let n2_Q15[k] be the normalized LSF coefficients decoded by the
// procedure in Section 4.2.7.5, n0_Q15[k] be the LSF coefficients
// decoded for the prior frame, and w_Q2 be the interpolation factor.
@ -404,14 +407,17 @@ func (d *Decoder) normalizeLSFInterpolation() error {
//
// n1_Q15[k] = n0_Q15[k] + (w_Q2*(n2_Q15[k] - n0_Q15[k]) >> 2)
if wQ2 := d.rangeDecoder.DecodeSymbolWithICDF(icdfNormalizedLSFInterpolationIndex); wQ2 != 4 {
return errUnsupportedLSFInterpolation
return nil, errUnsupportedLSFInterpolation
}
return nil
// TODO
n1Q15 = nlsfQ15
return n1Q15, nil
}
func (d *Decoder) convertNormalizedLSFsToLPCCoefficients(nlsfQ15 []int16, bandwidth Bandwidth) (a32Q17 []int32) {
cQ17 := make([]int32, len(nlsfQ15))
func (d *Decoder) convertNormalizedLSFsToLPCCoefficients(n1Q15 []int16, bandwidth Bandwidth) (a32Q17 []int32) {
cQ17 := make([]int32, len(n1Q15))
cosQ12 := q12CosineTableForLSFConverion
ordering := lsfOrderingForPolynomialEvaluationNarrowbandAndMediumband
@ -433,16 +439,16 @@ func (d *Decoder) convertNormalizedLSFsToLPCCoefficients(nlsfQ15 []int16, bandwi
// i'th entry of Table 28.
//
// https://datatracker.ietf.org/doc/html/rfc6716#section-4.2.7.5.6
for k := range nlsfQ15 {
i := int32(nlsfQ15[k] >> 8)
f := int32(nlsfQ15[k] & 255)
for k := range n1Q15 {
i := int32(n1Q15[k] >> 8)
f := int32(n1Q15[k] & 255)
cQ17[ordering[k]] = (cosQ12[i]*256 +
(cosQ12[i+1]-cosQ12[i])*f + 4) >> 3
}
pQ16 := make([]int32, (len(nlsfQ15)/2)+1)
qQ16 := make([]int32, (len(nlsfQ15)/2)+1)
pQ16 := make([]int32, (len(n1Q15)/2)+1)
qQ16 := make([]int32, (len(n1Q15)/2)+1)
// Given the list of cosine values compute the coefficients of P and Q,
// described here via a simple recurrence. Let p_Q16[k][j] and q_Q16[k][j]
@ -461,7 +467,7 @@ func (d *Decoder) convertNormalizedLSFsToLPCCoefficients(nlsfQ15 []int16, bandwi
qQ16[0] = 1 << 16
pQ16[1] = -cQ17[0]
qQ16[1] = -cQ17[1]
dLPC := len(nlsfQ15)
dLPC := len(n1Q15)
d2 := dLPC / 2
// As boundary conditions, assume p_Q16[k][j] = q_Q16[k][j] = 0 for all j < 0.
@ -510,7 +516,7 @@ func (d *Decoder) convertNormalizedLSFsToLPCCoefficients(nlsfQ15 []int16, bandwi
//
// https://datatracker.ietf.org/doc/html/rfc6716#section-4.2.7.5.6
a32Q17 = make([]int32, len(nlsfQ15))
a32Q17 = make([]int32, len(n1Q15))
for k := 0; k < d2; k++ {
a32Q17[k] = -(qQ16[k+1] - qQ16[k]) - (pQ16[k+1] + pQ16[k])
a32Q17[dLPC-k-1] = (qQ16[k+1] - qQ16[k]) - (pQ16[k+1] + pQ16[k])
@ -1032,6 +1038,35 @@ func (d *Decoder) limitLPCCoefficientsRange(a32Q17 []int32) {
}
}
// The prediction gain of an LPC synthesis filter is the square root of
// the output energy when the filter is excited by a unit-energy
// impulse. Even if the Q12 coefficients would fit, the resulting
// filter may still have a significant gain (especially for voiced
// sounds), making the filter unstable. silk_NLSF2A() applies up to 16
// additional rounds of bandwidth expansion to limit the prediction
// gain.
//
// https://datatracker.ietf.org/doc/html/rfc6716#section-4.2.7.5.8
func (d *Decoder) limitLPCFilterPredictionGain(a32Q17 []int32) (aQ12 []float64) {
aQ12 = make([]float64, len(a32Q17))
// However, silk_LPC_inverse_pred_gain_QA() approximates this using
// fixed-point arithmetic to guarantee reproducible results across
// platforms and implementations. Since small changes in the
// coefficients can make a stable filter unstable, it takes the real Q12
// coefficients that will be used during reconstruction as input. Thus,
// let
//
// a32_Q12[n] = (a32_Q17[n] + 16) >> 5
//
// https://datatracker.ietf.org/doc/html/rfc6716#section-4.2.7.5.8
for n := range a32Q17 {
aQ12[n] = float64((a32Q17[n] + 16) >> 5)
}
return
}
// Decode decodes many SILK subframes
// An overview of the decoder is given in Figure 14.
//
@ -1093,17 +1128,27 @@ func (d *Decoder) Decode(in []byte, isStereo bool, nanoseconds int, bandwidth Ba
nlsfQ15 := d.normalizeLineSpectralFrequencyCoefficients(bandwidth, resQ10, I1)
// https://datatracker.ietf.org/doc/html/rfc6716#section-4.2.7.5.4
d.normalizeLSFStabilization()
d.normalizeLSFStabilization(nlsfQ15)
// https://datatracker.ietf.org/doc/html/rfc6716#section-4.2.7.5.5
if err := d.normalizeLSFInterpolation(); err != nil {
n1Q15, err := d.normalizeLSFInterpolation(nlsfQ15)
if err != nil {
return nil, err
}
// https://datatracker.ietf.org/doc/html/rfc6716#section-4.2.7.5.6
a32Q17 := d.convertNormalizedLSFsToLPCCoefficients(nlsfQ15, bandwidth)
a32Q17 := d.convertNormalizedLSFsToLPCCoefficients(n1Q15, bandwidth)
// https://www.rfc-editor.org/rfc/rfc6716.html#section-4.2.7.5.7
d.limitLPCCoefficientsRange(a32Q17)
// https://www.rfc-editor.org/rfc/rfc6716.html#section-4.2.7.5.8
d.limitLPCFilterPredictionGain(a32Q17)
if signalType == frameSignalTypeVoiced {
return nil, errUnsupportedVoicedFrames
}
d.isPreviousFrameVoiced = signalType == frameSignalTypeVoiced
return
}

1
internal/silk/errors.go
View file

@ -7,4 +7,5 @@ var (
errUnsupportedSilkStereo = errors.New("silk decoder does not support stereo")
errUnsupportedSilkLowBitrateRedundancy = errors.New("silk decoder does not low bit-rate redundancy")
errUnsupportedLSFInterpolation = errors.New("silk decoder does not support LSF Interpolation yet")
errUnsupportedVoicedFrames = errors.New("silk decoder does not support voiced frames")
)