Natural Speech Coding Tools
Overview of the MPEG-4 Natural Speech Coding Tools
MPEG-4 Natural Speech Coding Tool Set [10] provides
a generic coding framework for a wide range of applications with speech signals.
Its bitrate coverage spans from as low as 2 kbit/s to 23.4 kbit/s. Two different
bandwidths of the input speech signal are covered, namely, 4 kHz and 7 kHz.
MPEG-4 Natural Speech Coding Tool Set contains two algorithms: HVXC (Harmonic
Vector eXcitation Coding) and CELP (Code Excited Linear Predictive coding).
HVXC is used at a low bitrate of 2 or 4 kbit/s. Higher bitrates than 4 kbit/s
in addition to 3.85 kbit/s are covered by CELP. The algorithmic delay by either
of these algorithms is comparable to that of other standards for two-way communications,
therefore, MPEG-4 Natural Speech Coding Tool Set is also applicable to such
applications. Storage of speech data and broadcast are also promising applications
of MPEG-4 Natural Speech Coding Tool Set. The specifications of MPEG-4 Natural
Speech Coding Tool Set are summarized in Tab. 1.
| HVXC | ||
|---|---|---|
| Sampling Frequency | 8 kHz | |
| Bandwidth | 300 - 3400 Hz | |
| Bitrate [bit/s] | 2000 and 4000 | |
| Frame Size | 20 ms | |
| Delay | 33.5 - 56 ms | |
| Features | Multibitrate Coding, Bitrate Scalability |
|
| CELP | ||
| Sampling Frequency | 8 kHz | 16 kHz |
| Bandwidth | 300 - 3400 Hz | 50 - 700 Hz |
| Bitrate [bit/s] | 3850 - 12200 28 Bitrates |
10900 - 23800 30 Bitrates |
| Frame Size | 10 - 40 ms | 10 - 20 ms |
| Delay | 15 -45 ms | 15 - 26.75 ms |
| Features | Mulibitrate Coding, Bitrate Scalability, Bandwidth Scalability |
|
MPEG-4 is based on tools each of which can be combined according to the user needs. HVXC consists of LSP (line
spectral pair) VQ (vector quantization) tool and harmonic VQ tool. RPE (regular pulse excitation) tool, MPE
(multipulse excitation) tool, and LSP VQ tool form CELP. RPE tool is allowed only for the wideband mode because
of its simplicity at the expense of the quality. LSP VQ tool is common in both HVXC and CELP. MPEG-4 Natural
Speech Coding Tools are illustrated in Fig. 6.
Figure 6: MPEG-4 Natural Speech Coding Tool Set 
Functionalities of MPEG-4 Natural
Speech Coding Tools
MPEG-4 Natural Speech Coding Tools are different from other existing speech coding standards such as ITU-T
G.723.1 and G.729 in the following three new functionalities: multibitrate coding (An arbitrary bitrate may
be selected with a 200 bit/s step by simply changing the parameter values), bitrate scalable coding,
and bandwidth scalable coding. Actually, these new functionalities characterize MPEG-4 Natural Speech Coding
Tools. It should be noted that the bandwidth scalability is available only for CELP.
Multibitrate Coding
Multibitrate coding provides flexible bitrate selection with the same coding
algorithm. It has not been available and different codecs were needed for
different bitrates. In multibitrate coding, a bitrate is selected among multiple
available bitrates upon establishment of a connection between the communicating
parties. The bitrate for CELP may be selected with as small a step as 0.2
kbit/s. The frame length, the number of subframes per frame, and selection
of the excitation codebook are modified for different bitrates [11].
For HVXC, 2 or 4 kbit/s can be selected as the bitrate.
In addition to multibitrate coding, bitrate control with a smaller step of
the bitrate is available for CELP by fine-rate control (FRC). In addition
to multibitrate coding, some additional bitrates not available by multibitrate
coding are provided by FRC. The bitrate may bedeviated frame by frame from
a specified bitrate according to the input-signal characteristics. When the
spectral envelope, approximated by the LP synthesis filter, has small variations
in time, transmission of liner-prediction coefficients may be skipped once
every two frames for a reduced average bitrate [12].
Scalable Coding
Bitrate and bandwidth scalabilities are useful for multicast transmission. The bitrate and the bandwidth can
be independently selected for each receiver by simply stripping off a part of the bitstream. Scalabilities
necessitate only a single encoder to transmit the same data to multiple points connected at different rates.
Such a case can be found in connections between a cellular network with mobile terminals and a digital network
with fixed multimedia terminals as well as in multipoint teleconferencing. The encoder generates a single common
bitstream by scalable coding for all the recipients instead of independent bitstreams at different bitrates.
The scalable bitstream has a layered structure with the core bitstream and enhancement bitstreams. The bitrate
control is performed by adjusting the combination of the enhancement bitstreams depending on the specified bitrate.
The core bitstream guarantees, at least, reconstruction of the original speech signal with a minimum speech
quality. Additional enhancement bitstreams, which may be available depending on the network condition, will
increase the quality of the decoded signal. HVXC and CELP may be used to generate the core bitstream when
the enhancement bitstreams are generated by TwinVQ or AAC. They can also generate both the core and the
enhancement bitstreams. Scalabilities in MPEG-4/CELP are depicted in Fig. 7.
Figure 7: Scalabilities in MPEG-4/CELP
Scalabilities include bitrate scalability and bandwidth scalability. These scalabilities reduce signal distortion
or achieve better speech quality with high frequency components by adding enhancement bitstreams to the core
bitstream. These enhancement bitstreams contain detailed characteristics of the input signal or components in
higher frequency bands. For example, the output of Decoder A in Fig. 7 is the minimum-quality signal decoded
from the 6 kbit/s core bitstream. The Decoder B output is a high-quality signal decoded from an 8 kbit/s
bitstream. Decoder C provides a higher-quality signal decoded from a 12 kbit/s bitstream. On the other hand,
the Decoder D output has a wider bandwidth. This wideband signal is decoded from a 22 kbit/s bitstream. The
high-frequency components of 10 kbit/s provides increased naturalness than Decoder C. Bandwidth scalability is
provided only by the MPE tool. The unit bitrate for the enhancement bitstreams in bitrate scalability is 2 kbit/s
for the narrowband and 4 kbit/s for the wideband. In case of bandwidth scalable coding, the unit bitrate for
the enhancement bitstreams depends on the total bitrate and is summarized in Tab. II.
| Core Bitstream (bit/s) | Enhancement Bitstream (bit/s) |
|---|---|
| 3850 - 4650 | 9200, 10400, 11600, 12400 |
| 4900 - 5500 | 9467, 10667, 11867, 12667 |
| 5700 - 10700 | 10000, 11200, 12400, 13200 |
| 11000 - 12200 | 11600, 12800, 14000, 14800 |