CMX608 datasheet(18/70 Pages) CMLMICRO

18 / 70 page

RALCWI Vocoder

CMX608/CMX618/CMX638

 2014 CML Microsystems Plc

D/608_18_38/11

CMX618/CMX638 in external CODEC mode, the SYNC output is not available, so another method needs

to be employed, e.g using a modulo 160 counter clocked by the CODEC frame sync.

Timed

It is assumed that the host has an accurate 20ms timer, derived from the master clock of the system,

which may or may not be synchronous with the audio CODEC. Wait for a timer event, instruct the device

to decode, and then supply a packet for decoding. For every subsequent timer event, supply another

packet for decoding. This is essentially the same as the event driven / method 2 above, the difference

being that if the timer is not synchronous with the CODEC then the device's slip management feature will

have to be employed. Further details of slip management are given in section 6.5.

The following diagrams show various aspects of the timed delivery of single-frame raw Vocoder packets

for decoding.

Figure 10 Single Frame Packet Decoding

Figure 10 shows the sequence of events including the initial decoder delay, which compensates for the

decoder's algorithmic jitter.

Figure 11 Single Frame Packet Decoding with Host Jitter

Figure 11 shows the effect of 4ms of host jitter with a default value of 8ms for the initial decoder delay.

After the second batch of samples there is a gap, where the worst case of early delivery followed by late

delivery has caused the CODEC to be starved of samples. To avoid this situation, the IDD has to be

increased.

CMX608 数据表(PDF) 18 Page - CML Microcircuits