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2004 Jan 26 20
Philips Semiconductors Product specification
MPEG-2 video and MPEG-audio/AC-3
audio encoder with multiplexer
SAA6752HS
7.3.5 BIT RATE CONTROL
The SAA6752HS supports two modes of video bit rate
control: variable bit rate and constant bit rate.
The Variable Bit Rate (VBR) mode is intended for burst
data transfer applications, where the bit rate is allowed to
vary but the image quality should be constant. In this
mode, a combination ofthree parameters can be set: Rvbr,
Qmin_VBR and Qmax_VBR. While aiming at the target bit
rate Rvbr, only quantizer scale values within the range
between Qmin_VBR and Qmax_VBR are applied.
Broadening this range leads to greater variations in picture
quality but better adherence to Rmax. Constriction of this
range forces a better constancy in picture quality at the
expense of meeting the target bit rate. Note that optimal
control results require reasonable combinations of Rmax,
Qmin_VBR and Qmax_VBR. Furthermore, the maximum
bit rate Rmax can be set. If Rmax is reached in VBR mode,
the CBR algorithm takes over the control by increasing the
quantizer scale values temporarily (over Qmax_VBR) to
guarantee that Rmax is never exceeded. Hence, the
closer Rmax and Rvbr are chosen, the more the control in
VBR mode turns to CBR mode behaviour.
The Constant Bit Rate (CBR) mode is intended for
applications, where a fixed channel rate is provided (e.g.
transmission systems). A tight control of the quantizer
scale is applied to make optimal use of the given
bandwidth. The parameter Rmax specifies the required
constant bit rate.
Independent of the bit rate mode (CBR or VBR), a B-frame
weighting factor (the weighting factor is applied to the
quantization scale) can be applied to further reduce the
bit rate of B-frames. In IP-only GOP structures, every
second P-frame is weighted by this factor generating
‘virtual B-frames’ to simulate a bit rate distribution similar
to IPB sequences. This feature can further improve the
perceptual rate-distortion ratio by taking advantage of the
inertia of the human visual system.
7.3.6 ADAPTIVE QUANTIZATION
Adaptive quantization is an algorithm that uses internal
generated statistics to fine tune the quantizer scale used
for encoding a specific macroblock. For example, the
controller adapts the quantization scale with respect to the
local complexity distribution within a frame, resulting in a
perceptually smoother picture quality. The amount of fine
tuning can be adjusted by control of the adaptive
quantization depth.
7.3.7 QUANTIZER MATRIX TABLE DOWNLOAD
The MPEG standard default quantizer matrices can be
overwritten to allow picture encoding optimization.
7.3.8 USER DATA INSERTION
User data insertion of up to 64 bytes is supported on GOP
and picture level.
Different modes can be selected via I
2
C-bus.
7.3.8.1 External user data insertion (permanently
repeated)
User data is downloaded via the I
2
C-bus to
subaddresses 73H/76H and the number of inserted user
data bytes is set via subaddresses 74H/75H. In Encode
mode the downloaded user data will be inserted
permanently into the user data area of the video stream.
It is possible to download a new set of user data during
Encode mode. The new data will be repeatedly inserted as
soon as the download is finished. It is possible to stop the
user data insertion with a special command.
7.3.8.2 External user data insertion (each downloaded
byte inserted only once)
In this mode each downloaded user data byte is inserted
only once into the user data area. If no new user data is
downloaded between two GOP or pictures then no user
data will be inserted. This mode can be used to transmit
more than 64 bytes of user data from the encoder to the
decoder, e.g. 1000 bytes distributed on 15 packets of
64 bytes and one packet of 40 bytes. The host has to
control the insertion and repetition of user data. A host
interrupt 'mode transition completed' is signalled, if not
masked and the bit 9 of the exception status word is set
when the user data have been read by the video encoder.
Then new user data can be downloaded via I
2
C-bus.