GS2972 Datasheet, PDF (57/125 Pages) Semtech Corporation – 3G/HD/SD-SDI Serializer with Complete SMPTE Audio & Video Support

4.7.16 Audio FIFO - HD and 3G
For HD and 3G formats, the audio FIFO block is a maximum of seven samples deep.
According to SMPTE ST 299, audio samples are multiplexed immediately in the next
HANC region after the audio sample occurs. A buffer size of seven samples takes into
account that there are no samples after the switching line (for one line) and the
worst-case video standard of 720p/24, plus a one-sample safety margin.
Due to the sample distribution used in HD video standards and the size of the buffer in
HD mode, no checking is made for buffer underflow/overflow conditions. The pointers
should maintain a variable offset between 0 and 6.
4.7.17 Five-frame Sequence Detection - SD
The GS2972 detects the frame sequence that describes the sample distribution for
synchronous audio.
The frame sequence is used in the generation of audio control packets; where the Audio
Frame Number (AFN) field describes the position of the current frame within the frame
sequence.
The frame sequence is also used in the generation of Audio Sample Distribution for
formats with 525 lines. Each frame has 1602 samples or 1601 samples, depending upon
the frame sequence.
The GS2972 sets the AFN of the Primary group control packets to zero, unless the
AFNA_AUTO host interface bit (400h bit 7) is set to produce automatic AFN generation.
The Multiplexer sets the AFN of the Secondary group control packets to zero unless the
AFNB_AUTO host interface bit (800h bit 10) is set to produce automatic AFN generation.
When the frame rate is 25Hz, every frame has 1920 samples and the AFN is always set
to one.
When the frame rate is 29.97Hz, an even number of samples (8008) are distributed over
five frames in the following sequence:
1602 1601 1602 1601 1602
The GS2972 sets the AFN field to a number between one and five, depending on where
the current frame lies within the sequence.
The GS2972 adds the offset specified in the AFN_OFS host interface field (400h bits 6-4)
to the generated AFN. The result of the addition wraps around such that the AFN will
always be in the range of one to five.
GS2972 3G/HD/SD-SDI Serializer with Complete
SMPTE Audio & Video Support
Final Data Sheet Rev. 9
GENDOC-047479 September 2013
www.semtech.com
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