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SAA7115_Datasheet.fm
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Philips Semiconductors
CS-PD Hamburg
CVIP2
Datasheet
SAA7115
Date:
10/23/01
Version:
0.67
8.7
Audio clock generation (subaddresses 30H to 3FH)
The SAA7115 incorporates with its Audio clock PLL (APLL), its second analog PLL (CGC2) the generation of multiple
different audio clocks for external usage. There are two basic modes for generating an audio clock (refer to figure 32):
Generating a frame locked Audio Master clock without using the second analog PLL (CGC2) (refer to chapter 8.7.1):
This frame locked audio clock is directly obtained from the digital Audio PLL and output at the device pin AMCLK (pin
37). Hence this signal carries the correct number of clock cycles per frame but still has a high frequency jitter. This
at the ASCLK pin (pin 39) - and a word select signal - output at the ALRCLK pin (pin 40).
Using this audio clock generation method audio clock frequencies it is not possible to generate frequencies of 384*fs
and 512*fs (fs = audio sampling frequency)
Generating a low jitter frame locked Audio Master clock supported by the second analog PLL (CGC2) (refer to chapter
8.7.2):
In this mode the digital Audio PLL output signal feeds the internal second analog PLL (CGC2) to remove high
frequency jitter from the audio clock signal. The resulting clock is output at the device pin AMCLK (pin 37).
This is already the audio clock for some high frequency audio clocks. All other audio clocks must be generated by
feeding back the AMCLK output signal into the AMXCLK input pin. The audio clock frequency will be defined by the
programming value of the SDIV[5:0] register (subaddress 38hex) and output at the ASCLK output pin (pin 39).
Both modes ensure that there is always the same predefined number of audio samples associated with a frame, because
the audio clock is locked to the frame frequency.
8.7.1
A
UDIO CLOCK GENERATION WITHOUT ANALOG
PLL (CGC2)
ENHANCEMENT
8.7.1.1
Master audio clock
The audio clock is synthesized from the same crystal frequency as the line-locked video clock is generated. The master
audio clock is defined by the parameters:
Audio master Clocks Per Field, ACPF[17:0] 32H[1:0] 31H[7:0] 30H[7:0] according to the equation:
Audio master Clocks Nominal Increment, ACNI[21:0] 36H[5:0] 35H[7:0] 34H[7:0] according to the equation:
See Table 28 for examples.
Remark
: For standard applications the synthesized audio clock AMCLK can be used directly as master clock and as
input clock for port AMXCLK (short cut) to generate ASCLK and ALRCLK. For high-end applications it is recommended
to either use the second CGC for audio clock generation by setting UCGC = 1 (see subaddress 3AH, bit 7) or use an
external analog PLL circuit to enhance the performance of the generated audio clock.
ACPF[17:0]
roundaudio master clock frequency
field frequency
-----------------------------------------------------------------------------
=
ACNI[21:0]
round-----------------------------------------------------------------------------
crystal frequency
2
23
×
=