Speex wideband mode (sub-band CELP)

For wideband, the Speex approach uses a quadrature mirror filter (QMF) to split the band in two. The 16 kHz signal is thus divided into two 8 kHz signals, one representing the low band (0-4 kHz), the other the high band (4-8 kHz). The low band is encoded with the narrowband mode described in section 8 in such a way that the resulting ``embedded narrowband bit-stream'' can also be decoded with the narrowband decoder. Since the low band encoding has already been described, only the high band encoding is described in this section.

Linear Prediction

The linear prediction part used for the high-band is very similar to what is done for narrowband. The only difference is that we use only 12 bits to encode the high-band LSP's using a multi-stage vector quantizer (MSVQ). The first level quantizes the 10 coefficients with 6 bits and the error is then quantized using 6 bits, too.

Pitch Prediction

That part is easy: there's no pitch prediction for the high-band. There are two reasons for that. First, there is usually little harmonic structure in this band (above 4 kHz). Second, it would be very hard to implement since the QMF folds the 4-8 kHz band into 4-0 kHz (reversing the frequency axis), which means that the location of the harmonics is no longer at multiples of the fundamental (pitch).

Excitation Quantization

The high-band excitation is coded in the same way as for narrowband.

Bit allocation

For the wideband mode, the entire narrowband frame is packed before the high-band is encoded. The narrowband part of the bit-stream is as defined in table 3. The high-band follows, as described in table 5. For wideband, the mode ID is the same as the Speex quality setting and is defined in table . This also means that a wideband frame may be correctly decoded by a narrowband decoder with the only caveat that if more than one frame is packed in the same packet, the decoder will need to skip the high-band parts in order to sync with the bit-stream.

Table 5: Bit allocation for high-band in wideband mode

Parameter	Update rate	0	1	2	3	4
Wideband bit	frame	1	1	1	1	1
Mode ID	frame	3	3	3	3	3
LSP	frame	0	12	12	12	12
Excitation gain	sub-frame	0	5	4	4	4
Excitation VQ	sub-frame	0	0	20	40	80
Total	frame	4	36	112	192	352

Table: Quality versus bit-rate for the wideband encoder

Mode/Quality	Bit-rate (bps)	Quality/description
0	3,950	Barely intelligible (mostly for comfort noise)
1	5,750	Very noticeable artifacts/noise, poor intelligibility
2	7,750	Very noticeable artifacts/noise, good intelligibility
3	9,800	Artifacts/noise sometimes annoying
4	12,800	Artifacts/noise usually noticeable
5	16,800	Artifacts/noise sometimes noticeable
6	20,600	Need good headphones to tell the difference
7	23,800	Need good headphones to tell the difference
8	27,800	Hard to tell the difference even with good headphones
9	34,400	Hard to tell the difference even with good headphones
10	42,400	Completely transparent for voice, good quality music