Re: Cassette port input frequency range? And CPU MHz ranges.

[Michael J. Mahon <mjmahon@aol.com>]

Egan Ford <datajerk@gmail.com> wrote:
> On Nov 27, 7:04 pm, mwillegal <m...@willegal.net> wrote:
>> Take a look at the sync signal used on the stock Apple 1 and Apple II
>> cassette interface on this page.
>> 
>> http://www.willegal.net/appleii/aci.htm
>> 
>> Though the circuit works as a zero crossing detector, there are clear
>> limits as to speed capability due to the relatively slow signal
>> transition times on the comparitor side of the input capacitor.
>> 
>> You may get a bit more out of it, by using a digital music player which
>> eliminates some the analog jitter involved with a real cassette player,
>> but I'd be surprised if you could get 10x improvement.
>> 
>> Regards,
>> Mike Willegal
> 
> Thanks Mike, that is what I was afraid of.  I'll run some tests in a
> week or two and try to measure the peak effective frequency for my //
> e.
> 
> Right now I have an effective baud rate of 1777 @ 8 bits giving me
> ~10x.  But that requires a sampling rate of 64K and a range of
> frequencies from 1032 to 32K.  It works in emulation, but not in the
> real world.

Exactly. And it wouldn't work in emulation either if it were a real
emulation!

The parameters for the Apple II cassette interface were chosen to
approximately optimize the speed for real-world audio recorders and tapes.

If one presumed that users had 200kHz instrumentation recorders, different
choices could have been made. 

As was pointed out, the "wow" of the tape transport is the primary
limitation on timing margins.  With a crystal-controlled time base (but
still asynchronous with the Apple clock), it would be possible to use a
byte-synchronous coding like NadaNet, and get 117k bits/sec. 

If timing variations are wider, it is necessary to synchronize more
frequently than once per byte, resulting in lower speed. 

All of this assumes that the data recovery is being done by code running at
1MHz on the Apple. 

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon