Date   : Sat, 27 Nov 2010 14:28:31 +1100
From   : awilliams@... (Alan Williams)
Subject: Electron Ferranti ULA reverse engineering progress

As the next  chip is the 6550, which is asynchronous I speculate that the
cassette interface is as well.  So data bytes on the tape are framed with
start & stop bits.  No clock recovery necessary.

Alan


-----Original Message-----
From: bbc-micro-bounces+awilliams=linkme.com.au@... [mailto:bbc-micro-bounces+awilliams=linkme.com.au
at lists.cloud9.co.uk] On Behalf Of James McGill
Sent: Saturday, 27 November 2010 11:37 AM
To: Pete Turnbull
Cc: bbc-micro@...
Subject: Re: [BBC-Micro] Electron Ferranti ULA reverse engineering progress

I (am probably) wrong, but I think you need a zero crossing detector as well,
to recover the clock in cases where you have a large number of 1's or 0's
in a row (and assuming the data isn't manchester encoded or similar).

Use of an external clock is insufficient because of variations in the speed
at which cassettes were played back.

On Sat, Nov 27, 2010@... AM, Pete Turnbull <pete@...>
wrote:
> Rick Murray wrote:
>
>> Speculation: Band-pass filter on the two freqs, presence of signal 
>> triggers a positive state for either logic one or logic zero, which 
>> is then clocked out as a stream of bits... That's my thought. It 
>> wouldn't need to be terribly complicated as the input is fairly well defined.
>> Just a bit of leeway on the freqs and clocking in order to cope 
>> gracefully with the minor variations in tape speed.
>
> Yup, some modems used to do that. ?Actually you only need one filter, 
> and a fairly narrow one at that -- presence of a tone at that 
> frequency is a logic zero ("space" state), anything else is a one 
> ("mark" or idle state). ?But some modems did have two filters, one for 
> each tone, in order to provide a carrier detect (either filter passing 
> signal = carrier present).
>
> --
> Pete ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Peter Turnbull
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Network Manager
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?University of York
>
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