4 Outline description

26.2903GPPAudio codec processing functionsExtended Adaptive Multi-Rate - Wideband (AMR-WB+) codecRelease 17Transcoding functionsTS

This TS is structured as follows:

Section 4.1 contains a functional description of the audio parts including the A/D and D/A functions. Section 4.2 describes input format for the AMR-WB+ encoder and the output format for the AMR-WB+ decoder. Section 4.3 presents a simplified description of the principles of the AMR-WB codec. In subclause 4.4, the sequence and subjective importance of encoded parameters are given.

Section 5 presents the functional description of the encoding functions of the AMR-WB+ extension modes, whereas clause 6 describes the decoding procedures for the extension modes. In section 7, the detailed bit allocation of the AMR-WB+ codec extension modes is tabulated. The AMR-WB speech modes are functionally unchanged as well as their bit allocation. Detailed information on them is found in [1].

4.1 Functional description of audio parts

The analogue‑to‑digital and digital‑to‑analogue conversion will in principle comprise the elements given below. In case of stereo codec operation, the given principles will be applied to the 2 available audio channels.

1) Analogue to uniform digital PCM

– microphone;

– input level adjustment device;

‑ input anti‑aliasing filter;

‑ sample‑hold device sampling at 16/24/32/48 kHz;

‑ analogue‑to‑uniform digital conversion to 16‑bit representation.

The uniform format shall be represented in two’s complement.

2) Uniform digital PCM to analogue

‑ conversion from 16‑bit uniform PCM sampled at 16/24/32/48 kHz to analogue;

‑ a hold device;

‑ reconstruction filter including x/sin( x ) correction;

‑ output level adjustment device;

‑ earphone or loudspeaker.

In the terminal equipment, the A/D function may be achieved

‑ by direct conversion to 14‑bit uniform PCM format;

For the D/A operation, the inverse operations take place.

4.2 Preparation of input samples

The encoder is fed with data from one/two input channels comprising of samples with a resolution of 16 bits in a 16‑bit word. The decoder outputs data in the same format and number of output channels. Though, mono output of decoded stereo signals is supported.

4.3 Principles of the extended adaptive multi-rate wideband codec

The AMR-WB+ audio codec contains all the AMR-WB speech codec modes 1-9 and AMR-WB VAD and DTX. AMR-WB+ extends the AMR-WB codec by adding TCX, bandwidth extension, and stereo.

The AMR-WB+ audio codec processes input frames equal to 2048 samples at an internal sampling frequency F_s . The internal sampling frequency is limited to the range 12800-38400 Hz, see section 8 for more details. The 2048-sample frames are split into two critically sampled equal frequency bands. This results in two superframes of a 1024 samples corresponding to the low frequency (LF) and high frequency (HF) band. Each superframe is divided into four 256-samples frames.

Sampling at the internal sampling rate is obtained by using a variable sampling conversion scheme, which re-samples the input signal.

The LF and HF signals are then encoded using two different approaches: the LF is encoded and decoded using the "core" encoder/decoder, based on switched ACELP and transform coded excitation (TCX). In ACELP mode, the standard AMR-WB codec is used. The HF signal is encoded with relatively few bits (16 bits/frame) using a bandwidth extension (BWE) method.

The basic set of rates are built based on AMR-WB rates in addition to bandwidth extension. The basic set of mono rates are shown in Table 1.

Table 1: Basic set of mono rates

Mono rate(incl. BWE) (bits/frame)	Corresponding AMR-WB mode
208	NA
240	NA
272	12.65
304	14.25
336	15.85
384	18.25
416	19.85
480	23.05

Note that in ACELP mode of operation, compared to AMR-WB, the VAD bit is removed, two bits per frame are added for gain prediction, and 2 bits are added for signaling frame encoding type. This adds 3 bits per frame. Note also that 16 bits/frame is always used for bandwidth extension (to encode the HF band). The first two basic mono rates are similar to other rates except that they use a fixed codebook with 20 bits or 28 bits, respectively.

For stereo coding, the set of stereo extension rates given in Table 2 are used.

Table 2: Basic set of stereo rates

Stereo extension rates (incl. BWE) (Bits/frame)
40	104
48	112
56	120
64	128
72	136
80	144
88	152
96	160

Note that the bandwidth extension is applied to both channels which requires additional 16 bits/frame for the stereo extension.

A certain mode of operation is obtained by choosing a rate from Table 1, in case of mono operation, or by combining a rate from Table 1 with a stereo extension rate from Table 2, in case of stereo operation. The resulting coding bitrate is (mono rate + stereo rate) Fs / 512.

Examples:

– For an internal sampling frequency of 32 kHz by choosing mono rate equal to 384 bits/frame and without stereo, we can obtain a bit-rate equal to 24 kbps and the frame length would be of a 16 ms duration.

– For an internal sampling frequency of 25.6 kHz by choosing mono rate equal to 272 bits/frame and stereo rate equal to 88 bits/frame, we can obtain a bit-rate equal to 18 kbps and the frame length would be of a 20 ms duration.

Note. The documentation of the AMR-WB+ floating-point C-code in [4] contains further information on how to use the executables compiled from this source code to exercise the various possible uses, in the codec, of mono bit rate, stereo bit rate and internal sampling frequency, and the resulting total bit rates.

4.3.1 Encoding and decoding structure

Figure 1 presents the AMR-WB+ encoder structure. The input signal is separated in two bands. The first band is the low-frequency (LF) signal, which is critically sampled at Fs/2 . The second band is the high-frequency (HF) signal, which is also downsampled to obtain a critically sampled signal. The LF and HF signals are then encoded using two different approaches: the LF signal is encoded and decoded using the "core" encoder/decoder, based on switched ACELP and transform coded excitation (TCX). In ACELP mode, the standard AMR-WB codec is used. The HF signal is encoded with relatively few bits using a bandwidth extension (BWE) method.

The parameters transmitted from encoder to decoder are the mode selection bits, the LF parameters and the HF parameters. The parameters for each 1024-sample super-frame are decomposed into four packets of identical size.

When the input signal is stereo, the Left and right channels are combined into mono signal for ACELP/TCX encoding, whereas the stereo encoding receives both input channels.

Figure 2 presents the AMR-WB+ decoder structure. The LF and HF bands are decoded separately after which they are combined in a synthesis filterbank. If the output is restricted to mono only, the stereo parameters are omitted and the decoder operates in mono mode.

Figure 1: High-level structure of AMR-WB+ encoder

Figure 2: High-level structure of AMR-WB+ decoder

4.3.2 LP analysis and synthesis in low-frequency band

The AMR-WB+ codec applies LP analysis for both the ACELP and TCX modes when encoding the LF signal. The LP coefficients are interpolated linearly at every 64-sample sub-frame. The LP analysis window is a half-cosine of length 384 samples.

4.3.3 ACELP and TCX coding

To encode the core mono signal (0-Fs/4 kHz band), the AMR-WB+ codec utilises either ACELP or TCX coding for each frame. The coding mode is selected based on closed-loop analysis-by-synthesis method. Only 256-sample frames are considered for ACELP frames (as in AMR-WB), whereas frames of 256, 512 or 1024 samples are possible in TCX mode.

ACELP encoding and decoding are similar to standard AMR-WB speech codec. The ACELP coding consists of LTP analysis and synthesis and algebraic codebook excitation. The ACELP coding mode is used in AMR-WB operation within AMR-WB+ codec.

In TCX mode the perceptually weighted signal is processed in the transform domain. The Fourier transformed weighted signal is quantised using split multi-rate lattice quantisation (algebraic VQ). Transform is calculated in 1024, 512 or 256 samples windows. The excitation signal is recovered by inverse filtering the quantised weighted signal through the inverse weighting filter (same weighting filter as in AMR-WB).

4.3.4 Coding of high-frequency band

Whereas the LF signal (0-Fs/4 kHz band) is encoded using the previously described switched ACELP/TCX encoding approach, the HF signal is encoded using a low-rate parametric bandwidth extension (BWE) approach. Only gains and spectral envelope information are transmitted in the BWE approach used to encode the HF signal.

The bandwidth extension is done separately for left and right channel in stereo operation.

4.3.5 Stereo coding

In the case of stereo coding, a similar band decomposition as in the mono case is used. The two channels L and R are decomposed into LF and HF signals. The LF signals of the two channels are down-mixed to form an LF mono signal, (0-Fs/4 kHz band). This mono signal is encoded separately by the core codec.

The LF part of the two channels is further decomposed into two bands (0-5Fs/128 kHz band) and (5Fs/128 kHz- Fs/4 kHz band). The very low frequency (VLF) band is critically down-sampled, and the side signal is computed. The resulting signal is semi-parametrically encoded in the frequency domain using the algebraic VQ. The frequency domain encoding is performed in closed loop by choosing among 40-, 80- and 160-sample frame lengths.

The high frequency part of the LF signals (Midband) are parametrically encoded. In the decoder, the parametric model is applied on the mono signal excitation in order to restore the high frequency part of the original LF part of the two channels.

The HF part of the two channels are encoded by using parametric BWE described below.

4.3.6 Low complexity operation

In the low complexity operation (use case B) the decision on the usage of ACELP and TCX mode is done in an open-loop manner. This approach introduces computational savings in the encoder.

4.3.7 Frame erasure concealment

When missing packets occur at the receiver, the decoder applies concealment. The concealment algorithm depends on the mode of the correctly received packets preceding and following the missing packet. Concealment uses either time-domain coefficient extrapolation, as in AMR-WB, or frequency-domain interpolation for some of the TCX modes.

4.3.8 Bit allocation

The bit allocation for the different parameters in the low-frequency band coding (Core) (0-Fs/4 kHz band) is shown in Tables 3, 4, 5, and 6. Note that there are two mode bits sent in each 256-sample packet. These mode bits are not shown in the bit allocation tables. The bit allocations for the stereo part is shown in Tables 7, 8, and 9. Note that there are also two additional mode bits for the VLF stereo encoder, which are not shown in the bit allocation. The bit allocation for the stereo HF part is by definition that of the bandwidth extension, as presented in Tables 7,8 and 9.

Tables 2 and 3 show the total bits per 256-sample packet, including mode bits.

Table 3: Bit allocations for ACELP core rates including BWE (per frame)

Parameter	Number of bits
Mode bits	2
ISF Parameters	46
Mean Energy	2
Pitch Lag	30
Pitch Filter	4 × 1
Fixed-codebook Indices	4 × 20	4 × 28	4 × 36	4 × 44	4 × 52	4 × 64	4 × 72	4 × 88
Codebook Gains	4 × 7
HF ISF Parameters	9
HF gain	7
Total in bits	208	240	272	304	336	384	416	480

Table 4: Bit allocations for 256-sample TCX window (Core)

Parameter	Number of bits
Mode bits	2
ISF Parameters	46
Noise factor	3
Global Gain	7
Algebraic VQ	134	166	198	230	262	310	342	406
HF ISF Parameters	9
HF gain	7
Total in bits	208	240	272	304	336	384	416	480

Table 5: Bit allocations for 512-sample TCX window (Core)

Parameter	Number of bits
Mode bits	2+2
ISF Parameters	46
Noise factor	3
Global Gain	7
Gain redundancy	6
Algebraic VQ	318	382	446	510	574	670	734	862
HF ISF Parameters	9
HF gain	7
HF Gain correction	8 × 2
Total in bits	416	480	544	608	672	768	832	960

Table 6: Bit allocations for 1024-sample TCX window (Core)

Parameter	Number of bits
Mode bits	2+2+2+2
ISF Parameters	46
Noise factor	3
Global Gain	7
Gain redundancy	3+3+3
Algebraic VQ	695	823	951	1079	1207	1399	1527	1783
HF ISF Parameters	9
HF gain	7
HF Gain correction	16 × 3
Total in bits	832	960	1088	1216	1344	1536	1664	1920

Table 7 Bit allocations for stereo encoder for 256-sample window

Parameter	Number of bits
Mode bits	2
Global Gain	7
Gain	7
Unused bits	1
Midband	6							12
Algebraic VQ	1	9	17	25	33	41	49	51	59	67	75	83	91	99	107	115
HF ISF Parameters	9
HF gain	7
Total in bits	40	48	56	64	72	80	88	96	104	112	120	128	136	144	152	160

Table 8 Bit allocations for stereo encoder for 512-sample window

Parameter	Number of bits
Mode bits	2+2
Global Gain	7
Gain	7
Unused bits	1+1
Midband	6×2							12×2
Algebraic VQ	16	32	48	64	80	96	112	116	132	148	164	180	196	212	228	244
HF ISF Parameters	9
HF gain	7
HF Gain correction	8 × 2
Total in bits	80	96	112	128	144	160	176	192	208	224	240	256	272	288	304	320

Table 9 Bit allocations for stereo encoder for 1024-sample window

Parameter	Number of bits
Mode bits	2+2+2+2
Global Gain	7
Gain	7
Unused bits	1+1+1+1
Midband	6×4							12×4
Algebraic VQ	46	78	110	142	174	206	238	246	278	310	342	374	406	438	470	502
HF ISF Parameters	9
HF gain	7
HF Gain correction	16 × 3
Total in bits	160	192	224	256	288	320	352	384	416	448	480	512	544	576	608	640