SAA7205 Philips Semiconductors, SAA7205 Datasheet - Page 36

• Manufacturer Part Number: SAA7205
• Description: MPEG-2 systems demultiplexer
• Manufacturer: Philips Semiconductors

Philips Semiconductors
7.12
Time stamp processing generates decoding (DTS) or
presentation (PTS) start interrupts for source decoders
(bits: ‘irpt_audio_strt’, irpt_video_strt’, address 0x0000,
see Table 13). Each time the stamp processor therefore
compares emulated PTS/DTS values (word:
‘video_emu_pts’, addresses 0x0505 and 0x0506, or
‘audio_emu_pts’, addresses 0x0605 and 0x0606,
see Table 13) to the local system time clock (STC,
see Fig.20). An interrupt (IRQ) to the microcontroller is
generated in the event of a positive zero transition of the
differences (STC - ‘video_emu_pts’ and STC -
‘audio_emu_pts’).
Interrupt-handling routines in the microcontroller translate
the demultiplexer interrupt to control and synchronization
data for the attached source decoder, as illustrated in
Fig.23 for the video time stamp processor. Figure 23
assumes that PTS/DTS are retrieved inside the video
decoder, but this is not necessary. The demultiplexer also
retrieves PTS/DTS words from the stream (words:
‘video_pts’, ‘video_dts’, addresses 0x0501 to 0x0504,
see Table 13). In contrast to what is illustrated in Fig.23,
video PTS/DTS processing could therefore be identical to
audio PTS/DTS processing (see Fig.24).
While the third party video decoder could retrieve
PTS/DTS data from the incoming PES stream, the audio
decoder generally does not. PTS/DTS retrieval is therefore
performed in each of the time stamp processors
1997 Jan 21
handbook, full pagewidth
MPEG-2 systems demultiplexer
Time stamp processing (PTS/DTS)
MICROCONTROLLER
(LOOP FILTER)
Fig.21 VCO control for local time reference regeneration.
DMUX
CCLKI (27 MHz)
36
PWMO
VO
(audio and video) within the demultiplexer. It is for the
microcontroller to decide whether it uses the retrieved time
stamps. For audio time stamp processing the
microcontroller may want to use the values retrieved by the
demultiplexer (words: ‘audio_pts’, audio_dts’, addresses
0x0601 to 0x0604, see Table 13) when operating in the
software controlled synchronization mode. In this mode
(bit ‘ c_sw_sync’ = 1, address 0x060A, see Table 13) the
microcontroller loads emulated PTS values into the
demultiplexer (words: ‘audio_emupts’, addresses 0x0605
to 0x0606, see Table 13) to get it to generate start
interrupts (interrupt: ‘irpt_audio_strt’, address 0x0000,
see Table 13), as illustrated in Fig.23. However, audio
synchronization can also be performed automatically by
the demultiplexer (bit ‘ c_sw_sync’ = 0, address 0x060A,
see Table 13) (see Section “Interfacing to SAA2500 and
third party audio decoders”).
The microcontroller has to perform time stamp emulation
on the basis of incoming PTS/DTS values (words:
‘audio_pts’, ‘audio_dts’, addresses 0x0601 to 0x0604,
see Table 13). Emulation involves compensation for
source decoder internal delays and repetitive generation
of time stamps. The latter could be necessary because
time stamps could be needed for every access unit in an
elementary stream, but are broadcast far less frequently.
It should be noted that video PTS/DTS processing can
operate along the same lines as illustrated in Fig.23 for
audio decoders.
R
C
control voltage
OSCILLATOR
MGG783
Preliminary specification
SAA7205H