Date   : Fri, 11 Feb 2011 21:40:13 +0000
From   : pete@... (Pete Turnbull)
Subject: Fwd: Would it make sense to have another cassette

On 11/02/2011 21:05, nicola giacobbe wrote:
> Hello to everyone,
> I have a small question only the greatest BBC masters could answer.
> I'd like to save the wav files of many beeb programs for future
> compatibility but, even with today's storage they are unnecessarily
> redundant to be easily kept. Instead of relying on the future grown of
> storage I would like to be able to recreate them on the fly. I mean, the
> cassette files for the BBC micro are only a succession of 1200 and 2400 Hz
> tones so I could just store the tone (1 bit) and duration and let the
> computer recreate it. That could drastically reduce the space needed.
> What do you think about? It is feasible or are there caveats that could
> hinder the effort even before starting (like: program protections no
> longer working or non-standard tones used?)

A couple of suggestions come to mind.

Firstly, it's just a serial bitstream where each one or zero is 
represented by an exact number of cycles of 1200/2400Hz, so why not just 
store the original bitstream?  One way to do this would be to get at the 
digital bitstream before it's turned into tones; I do exactly this on 
other machines where it's easy to get at the digital ones and zeros 
before the tone encoder.  I haven't tried it on a Beeb, but I suspect 
it's reasonably easy to tell the serial processor to output via the DOUT 
line to RS423 line instead of the CASOUT line that drives the audio 
output on the cassette port -- that's determined by a bit in the serial 
processor's control register.  The serial processor is driven by the 
6850, so the data bytes are already surrounded by start and stop bits 
and you could capture it on a PC (or other device) serial port.  That's 
exactly what I do for my Exidy Sorcerer, and I can store the file son 
disk and replay them into the target machine (my Sorcerer in this case).

Secondly, if it's just a question of saving space, convert the WAV files 
to MP3s for storage and playback.  That works really well; I use my iPod 
for demos on many "classic" computers, including Beebs.  An MP3 at even 
modest bitrates is easily good enough for reliable storage.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York