80 $ The cheapest usb audio interface for Pro Tools:

If you’re just getting starting in music production, sound engineering or just recording your band’s sample cd, the few basic important things that you need are:
1> Computer with multiple processors (I’ll discuss that later)
2> Studio monitors or headphones
3> A digital audio workstation with the midi interface for your midi instruments
4> Audio interface.

The smart people at Digidesign/Avid made their digital audio workstation in such a way that it won’t work without a Pro Tools compatible audio interface hard ware. I have no idea why they did it this way, the possible reasons could be market domination, profit, sales, or maybe quality maintainability. That’s a big thing to debate about.

Now I’m here to discuss the cheapest available interface to run pro tools, that is the M-Audio Transit 

This costs a mere 79.99 $ and does what its supposed to, gives you a 24 bit/96KHz audio output and makes pro tools work !
Very easy to set this up, simply plug it in your usb port and that’s it !
It has :
Line/Optical Input
Optical Output
Headphone/Line Output
So you can record stuff at 24bit/96KHz, it also has Ableton Live Lite 4 music production software.
I have also seen many music lovers use this to get the most out of their head phones/sound system.

 


Dimensions: 2.2 x 3.6 x 0.9 in.
Weight: 1.6 ounces
Making it the smallest portable pro tools interface after the Digidesign Mbox 2 Micro Usb
The Mbox 2 micro is for about 250 $ !
Thus the cheapest audio interface The M-Audio Transit supports Pro Tools M-Powered, Its just same as the Pro Tools LE, also notice that the Mbox micro 2, and Mbox mini 2 output at 24bit/48KHZ.
Thus, the cheapest 24bit/96KHZ audio interface!

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System  Requirements:
PC:
UDMA, EIDE or HDD
Windows 98 SE/ME/2000/XP
96kHz Operation: Pentium III 500 mHz, 128 MB RAM
48 kHz Operation: Pentium II 400 mHz, 64 MB RAM
Mac:
OS 9.1 (9.2 recommended)
OS 10.1 or higher*

96kHz Operation: G3 or G4 w/ native USB, 128 MB RAM
48kHz Operation: G3 or G4 w/ native USB, 64 MB RAM

Best of luck to all of you!