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we had a discussion about several HLB-related subjects:
We repeated the experiment to measure the effective user-transfer-rate.
Results were nearly the same - impossible to transfer 13000 bps when
using async-modem-mode (14.4 carrier), even with V42 enabled.
However we do not want to use the modems in sync-mode.
Main reasons for that are:
- Difficult to implement on standard-pc
- No real need to be compatible with 'VP1'
- Might not be supported (the same way) with any 14.4-modem
So our preferred solution to the bandwidth-problem is:
- We use 14.4-modems (minimum), std-gsm-codec, async-protokoll.
- To reduce data-output we either reduce audio sample-rate
(still running gsm-codec the same way) or we 'drop frames'
from time to time. The first method is preferred, because
is provides continuous audio. The second method might have
to be supported anyway when dealing with internet-frames.
We are still in urgent need for a parallel-bus (uP-Bus) compatible
AD/DA-converter-chip. There is one option "AD1875" (nr may be wrong),
but it is unclear wether price, avaiability etc. is ok for us.
Using such a chip would simplify design and give us another serial
port for free (would be used for AD/DA otherwise).
Any hint about other chips of this kind would be very nice,
as we could finish hardware-design and start building prototypes !
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