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Subject: Re: USB Design. Need advice.
Date: 27 Dec 2002 20:43:15 -0600
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"Dan McDaid" wrote in
> Hi. I need to make an isochronus usb device that has two audio out
> channels and two in.
> I'm new to USB but have a decent background in electronics.
> My question is pretty simple. I can rig up a USB-DAC IC to get a
> singe audio out channel. Now if i use two of these ICs and put them
> behind a USB-Hub IC then with appropriate drivers I can send the
> digital streams to each IC, which are essentially separate devices.
> Similarly, I can rig up two ADC-USB circuits and put them behind this
> same internal hub. Bang it all in a box, stick some phono plugs on
> and a usb cable and with any luck I can get down to writing the
> This all seems pretty sloppy though and there must be a way I can
> combine all this stuff into one device without this whole hub idea.
> Trouble is, Ive never used USB before and should really do some
> reading. Theres so much on the net though its hard to wade through
> all the irrelevant stuff.
> Im planning to build my own version of www.finalscratch.com. The
> idea is, two vinyl turntables are plugged into the 2x IN channels, the
> two out channels get connected to the DJ mixer console. The two vinyl
> disks that are put onto each turntable respectively, are pressed with
> some sort of waveform that can be interpreted digitally as a timeline.
> (waiting on a replacement LP from Stantons FinalScratch - in post so I
> can see how it works and rip it off - cheaper than having my own vinyl
> pressed several times until I got the right idea!). The idea is that
> two mp3s are streamed out the interface into the mixer. The timebase
> of each mp3 is mapped to and controlled by the vinyl. Thus you can
> manipulate mp3s exactly like vinyl.
> Why do I want to do this? Well apart from the $500 price tag for the
> original FinalScratch, its an interesting project.
USB is a pain as far as I could tell when designing things for myself...
although I've never had to transfer as much data as you are... I have
only ever made the device for a COM port... then used a com-usb IC.
also another thing to consider....
is USB going to be fastenough if you do this?
sorry I don;t remember... but if I remember correctly...
12Mbps is theoretical speed
and lucky to get even 7Mbps in actual usage
so 7Mbps over 4 IC's (as you suggested) including loss in that IC will
bring you to about 1.5Mbps, 1500kbps ... or 187.5kBps ..... which because
we are going on the safe side of loss... would only Just be anough to
transfer a decent qaulity MP3 (190kBps)
hmmm..... just a view.
may be wrong... but looking at it that way... you would need to compress
your music, before sending it to the computer...
there are easy to use cheap MP3 decoding/encoding IC's around with nicely
matching DAC/ADC's .... then just interface it to USB.
but I suggest you try something else...
like... making a controller card for a PCI slot... or USB2.0, or IEE1394.
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