Tapas Das
Transfer rates for USB/FireWire/MIDI
Tue Sep 18 19:54:04 2001


Hello Wily,

Welcome to the world of digital audio! Trust me, everyone had
to go through this phase of confusion and learning before
getting a single sound byte recorded on the PC.

Let's start with the data transfer rate of common interfaces.

The slowest (and the most common and cheapest) is the Serial port
interface. A high speed serial interface has a maximum transfer
rate of 120 Kilo bits per second (Kpbs) = 0.12 Mega bits per second (Mbps).

Right off the bat, let me make an important distinction which is
often confused. In computer parlance, 8 bits make a byte.
Bits are abbreviated with a lower case "b", while Bytes are abbreviated
with an uppercase "B". 

For example, 80 Mega bits per second (80Mbps) is the same as
10 Mega byte per second (10MBps).

From now for sake of conformity I will state all speeds in Mega bits per second.

A Enhanced parallel port interface (ECP/EPP) is 10 times faster
than a serial interface. It ran reach speeds up to 1.2Mbps.

Next up the speed chain is the original USB version 1.1 serial interface.
The transfer rate is 12 Mbps. This is 10 times faster than Parallel
and 100 times faster than your Serial interface.

The recent USB version 2 interface is around 425Mbps, about 35 times
faster than USB 1.1

The IEEE 1394 FireWire interface is about 400Mbps and there is work
on finalizing FireWire version 2 which will be significantly faster.

The speed champ is a high speed SCSI interface topping everybody
at 1200Mbps, about 3 times faster than SCSI.

So in a nutshell we have:

Serial 0.12Mbps
Parallel 1.2Mbps
USB 1.1 12Mbps
USB 2 425Mbps
FireWire 400Mbps
SCSI 1200Mbps

So how do these transfer speeds relate to MIDI and Digital Audio?

MIDI is a serial protocol. It is theoretically impossible to 
trigger all 3 notes in a chord at the exact same time. This is because
data cannot be transferred in parallel. The instructions go out in
series, one at a time. The speed for MIDI is about 31000 bytes per second
or 31Kbps or 0.031Mbps. In practice the delay or "latency" is very very
small, measured in milliseconds.

A serial port with a maximum transfer rate of 0.12Mbps is more than 
adequate to handle MIDI streams.

MIDI routers and patch bays which have to handle multiple MIDI channels
like the MOTU MTP-AV use High Speed Parallel interfaces for increased
bandwidth.

How about Digital Audio. It you take the digital data coming off a
standard red book CD (2 channel stereo - 44.1KHz @ 16 bits per second),
we get a data throughput of 2 x 44,100 x 16 = 1,411,200 bits per second
or 1.41Mbps.

This data volume far outweighs MIDI streams. When you are recording
and playing back multiple channels of Digital Audio and simultaneously
trying to squeeze in a MIDI stream, it creates bottlenecks for
a USB interface. You make experience a slight delay or "latency" which
is noticeably more than the regular MIDI latency.

This is unavoidable. Perfect synch requires data to arrive simultaneously.
USB (Universal Serial Bus) is a serial interface. Data travels single file.

USB was never intended for Digital Audio. It was designed for easy
connection of peripheral devices with lower data rates. FireWire, 
invented and marketed by Apple/Sony, was designed grounds up for
transferring Digital Audio and Video and remains the interface of choice
for professionals. However, USB 2.0 may change all that.

Where extreme speeds and reliability are of paramount importance, high
speed SCSI is still the way to go. They are used in storage devices
and servers.

Having said that, you should be fine with the Tascam US-428)!

Tapas.