[sdiy] Single chip digital delay

[Seb Francis]

And talking of a delay using a simple home-rolled 1-bit oversampling 
ADC/DAC and some FIFO memory .. you can get PICs with 3968 bytes ~= 
32kbit of memory.  These run up to 40MHz which gives an instruction 
cycle rate of 10MHz.  It ought to be easily possible to perform the FIFO 
operation in 10 instructions per bit, with plenty of spare instructions 
left over to read the PIC-ADC input from a control pot to set the delay 
length.  So a 1MHz sampling rate would give a high quality delay of up 
to 30ms or so.

This way of setting the delay length also has the big advantage that the 
audio quality stays constant as the delay length is changed (as opposed 
to changing the delay time by changing the clock frequency).  And of 
course when keeping the clock frequency constant one doesn't have to 
worry about switching capacitor values in the ADC/DAC integrators.

Of course the 'low-fi' effect obtained by slowing the clock frequency 
wouldn't be obtained, but hey I have a BBD based echo box already for 
this kind of effect.  And actually it would be easily possible to vary 
the clock frequency of the PIC down to obtain much longer delays (and 
low-fi effects).

.. anyway, just thinking out loud :)

Seb



Seb Francis wrote:

> (CCing back onto the list in case the info is of interest)
>
>
> Hi GB,
>
> Thanks for these links.  The 2nd one looks interesting - I hadn't 
> thought of making my own 1-bit oversampled ADC/DAC and just stringing 
> some FIFO memory in between.  I think your calculations for the memory 
> storage are not quite right though: should simply be the sampling rate 
> * the time to give the number of bits needed.  But for this kind of 
> conversion it probably needs a sampling rate of somewhere near 1MHz is 
> needed to get decent quality .. so 10ms delay = ~10kbit (pretty much 
> the same result as calculating it based on 'normal' sampling rate and 
> 16bit resolution).
>
> The first link mentions the Mitsubishi M65830 IC and that it has been 
> discontinued.  I think the replacement would be the M65850 which is 
> now sold by Renesas who took over some of Mitsubishi's product line in 
> 2003.  I've been finding these chips quite difficult to get hold of, 
> but hopefully (fingers crossed) a distributor is going to send me some 
> samples in a couple of weeks.
>
> Seb
>
>
>
> MTG wrote:
>
>> That's an interesting question.  Here are a couple of links
>> of possible interest ;-)
>>
>> http://sound.westhost.com/project26.htm
>>
>> http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=8056
>>
>> For a digital storage method, ou need to define your memory 
>> requirements.  EX: 44.1 kilosamples/sec * .010s * 16 bits = 7kbits
>>
>> so it wouldn't be too hard to do (memory-wise).
>> That being said a one-chip DSP might do it or
>> even a fast microcontroller.
>>
>> GB
>>
>>
>> -----Original Message-----
>>  
>>
>>> From: Seb Francis <seb at burnit.co.uk>
>>> Sent: Jan 24, 2006 3:54 AM
>>> To: Synth DIY <synth-diy at dropmix.xs4all.nl>
>>> Subject: [sdiy] Single chip digital delay
>>>
>>> On the subject of delays.. I've been wanting to build a simple 
>>> digital delay (or two) for the purpose of making a wide stereo image 
>>> .. e.g. hard pan the original sound to one side and hard pan a few 
>>> ms delay of the sound to the other side.
>>>
>>> The delay needs to be clean and accurate (so BBDs are out!), and the 
>>> delay time doesn't need to be very long.  I guess what I need is 
>>> something like a good quality single chip digital delay.  Anybody 
>>> know of something suitable?
>>>
>>> Seb
>>>
>>>
>>>   
>>
>>
>>
>>
>>  
>>
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