GS2971A Datasheet, PDF (89/152 Pages) Gennum Corporation – Integrated audio clock generator

Frame 5
Frame 1
Frame 2
8008 Samples
Frame 3
Frame 4
Frame 5
Frame 1
1602
1602
1601
1602
1601
1602
1602
1602 Repeat
Figure 4-45:Sample Distribution over 5 Video Frames (525-line Systems)
When the audio inputs are asynchronously switched or disrupted, the audio
de-embedder continues to write audio samples into the audio buffer, based on the
current five-frame sequence. The de-embedder then re-locks to the new five-frame
sequence, at which point a sample may be lost.
NOTE: In SD, all four channel pairs must follow the same five-frame sequence.
4.19.3.3 Audio FIFO Block
The function of the FIFO block is to change the audio data word rate from the ANC rate
multiplexed with the video signal to the 48kHz audio output rate.
The audio FIFO block contains the audio sample buffers; one per audio channel. Each
buffer is 36 audio samples deep. At power up or reset, the read pointer is held at the zero
position until 26 samples have been written into the FIFO (allows for 6 lines per frame
with no audio samples; a maximum of 4 samples per line in SD Mode). See Figure 4-46.
Read
Pointer
Address 0
25
35
Audio Buffer
Write
Pointer
Figure 4-46:Audio Buffer After Initial 26 Sample Write
The position of the write pointer with respect to the read pointer is monitored
continuously. If the write pointer is less than 6 samples ahead of the read pointer (point
A in Figure 4-47), a sample is repeated from the read-side of the FIFO. If the write pointer
is less than 6 samples behind the read pointer (point B in Figure 4-47), a sample is
dropped. This avoids buffer underflow/overflow conditions.
GS2971A 3Gb/s, HD, SD SDI Receiver
Data Sheet
54311 - 2
September 2012
89 of 152