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17. When I travel internationally can I take
my ISA 220 with me?
No problem. The power supply is a multi-tap design, so all you need to do
is turn the fuse holder around (and, if necessary, change the fuse) to change
the voltage to match whichever country you are in.
18. Is there an optional digital output card?
Yes. The card offers a stereo A/D converter, with 24/96 spec (selectable
options from front panel buttons.) The single card fits into the main ISA
220. The card includes a Wordclock input and, uniquely in the processing
world, a Digidesign Pro Tools™ Superclock direct connection via an
additional BNC connector, so that the unit can be slaved to an external
master clock. It allows 16, 20 or 24 bit operation at 44.1, 48, 88.2 and 96
kHz sample rates. Digital audio formats supported are S/PDIF (on single
RCA connector), AES-EBU (on single XLR connector) and optical. The
card can be ordered as part of a ‘digital-ready’ ISA 220 when the main unit
is purchased, or can be retrofitted later. So if you want to take the output
of the ISA 220 to a digital system which doesn't support 24bit 96 kHz, no
problem; you can select any of the range of different clocks and bit rates as
well as choosing to use an external clock source, from the front panel.
19. Why is a Superclock input important?
If a customer has a Pro Tools TDM system and wants to lock it to an
external analogue multi-track (s)he needs a USD, (Universal Slave Driver,
Digidesign’s premier sync box.) This box looks at the speed of incoming
timecode and then varies the Superclock frequency up and down to match.
Therefore, because the Superclock is basically 256 x the speed of
wordclock, the playback or record speed of Pro Tools is matched (very
accurately) to the machine's speed and any attached Digidesign Audio
interfaces will also be adjusted.
If the customer now wants to record off the multitrack into Pro Tools via
an ISA box, they have a problem if they don’t have a Superclock input,
because the ISA would be running off its own internal crystal, and not
looking at the speed information being calculated by the USD. It would be
running at precisely 44.1 or 48k with a very high stability, however the
analogue deck would be ‘wowing and fluttering’ all over the show.
Therefore by providing a Superclock input, you can use the USD to clock
the ISA module, and therefore lock the ISA up to anything you are locking
Pro Tools up to.
Also any TDM Pro Tools equipped with a USD can be switched into
varispeed mode. Using Pro Tools’ Session Setup window, a slider allows
the overall speed of Pro Tools to be moved up or down. This is achieved
by telling the USD to adjust its internal clock and therefore its Superclock
output. This varied Superclock output then feeds any Digidesign interface
as above. So if a customer wants to use an ISA 220, but at the same time
use varispeed in Pro Tools, they need a Superclock input.
20. Is there an optional digital input card?
No, because all the processing in the ISA 220 is entirely analogue - so even
if there were a digital input, the digital signal would have to be immediately
pass through a D/A converter to allow processing!
21. Does the card include dithering?
Yes, the word length of a 24 bit input can truncated down to 20 or 16 bits
and then dithered prior to digital output.
22. Why are the Int A/D and Ext A/D inputs
fed to the digital output card via the
Limiter?
The input to the A/D converter must not exceed 0dBFS in order to
prevent digital clipping. The limiter therefore comes after the A/D
converter inputs so that the user is protected from digital clipping.
23. Can I lock directly to Pro Tools from
the digital output of the ISA 220?
Yes, the digital output board is designed so that it can synchronise to
external wordclock signals, or to Digidesign's Superclock.
24. Why is 24 bit 96 kHz specification
important?
An A/D converter works by sampling the audio waveform at regular points
in time, and then quantising those values into a binary number, which
relates to the number of bits specified. The quantised signal must then be
passed through a D/A converter before it becomes audible. In simple
terms, the D/A essentially ‘joins the dots’ plotted by the A/D converter
when the signal was first converted to digital. The number of dots to join,
combined with how little those dots have been moved, determines how
accurate the final signal will be compared to the original. The greater the
sample rate and bit rate, the more accurate the whole digital process is. So
24 bit/96 kHz performance will ensure more accurate digital transfer of
your audio information compared to the old 16 bit/44.1 kHz standards.
(You can still use these standards for compatibility reasons if you need to,
as the ISA 220 also allows 16 bit/44.1 kHz operation.)
25. Can I retrofit a digital board to an
analogue ISA 220 at a later date?
Yes, and you can do it yourself - it can easily be retro-fitted by the
customer without any soldering etc, just a few screws to undo, and one
clip-connector to join to the main PCB.
26. How would you use the ‘Int A/D direct’
and ‘Ext A/D direct’ inputs on the rear
panel?
The ‘Internal A/D Direct’ input is used to route a signal directly to the A/D
card via the Limiter. It replaces the ‘Internal’ signal feed to the A/D card,
(i.e. whatever is connected to the mic, line or instrument input.) The
External A/D Direct’ input is also used to route a signal to the optional
A/D board card via the Limiter. The signal is fed to the ‘spare’ side of the
A/D card not being used by the internal signal, and so does not replace it.
27. What are the differences between the
ISA 430 and the ISA 220?
The ISA 430 includes an Expander/gate section with sidechain access,
sidechain monitoring, and external keying, and an Insert point which can be
post mic pre/pre EQ, post EQ/pre-compressor, or post compressor/pre-
limiter. The ISA 430’s EQ is the original Focusrite EQ with 6 frequency
selections for shelving EQ, and frequency-conscious compression and
gating (EQ feeding the sidechain of the compressor or gate) is possible.
The ISA 430’s Split function allows it to be used as two separate channels
for discrete EQ and compression simultaneously. The ISA 430 features
both input AND output transformers (however, note that the ISA 220’s
input transformer also applies to the line input, whereas the ISA 430
transformer is for the mic input only.) There’s also a Post mic pre output
on the rear panel, and the ISA 430’s Instrument input has 10dB more range
(0-40dB on the ISA 430,) but the ISA 430 lacks the ISA 220’s blend
function.
