ENGLISH
12
Q: Is the ISA 428 a Class A device, and why is that
important?
A: Yes, the ISA 428 is a Class A device. Why? Class A is a
type of amplifier design in which you have a standing DC
current running through your amplifier circuits all the time.
As the signal comes along you vary what you're taking from
that, rather than switching between supplying a positive
current for one half of the waveform and a negative current
for the other half. This results in the ability to represent
audio in a more linear (distortion free) manner all the way
through the circuit. Cheaper processors use IC amplifiers
which run close to Class B and don't have the same standing
DC current, which means the transistors inside the chips
switching off and on, inevitably resulting in a less linear
performance.
Q: Should balanced connectors be used with the ISA
428?
A: Yes, where possible. Alternatively, if using an unbalanced
instrument source, you can connect to the four unbalanced
1/4" inputs.
Q: What's the ISA 428's bandwidth? Does it have
the same kind of spectacular bandwidth that has
given the Red and ISA range units their reputation
for ‘open-ended’ sound?
A: Yes. The bandwidth is the same as the classic Focusrite
units of old: 10Hz-200kHz!
Q: Is there an optional digital input card?
A: No, because the ISA 428 is primarily a 'front end'
product. In other words, the only devices which are likely
to be connected to the 428's inputs are analogue sound
sources such as microphones, guitars etc.
Q: Why is the 24-bit 192kHz specification
important?
A: An A/D converter works by sampling the audio
waveform at regular points in time, and then quantizing
those values into a binary number, which relates to the
number of bits specified. The quantized 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/192kHz performance
will ensure more accurate digital transfer of your audio
information compared to the old 16-bit/44.1kHz standards.
This is especially important if further digital signal processing
is to be applied to the signal once converted to digital, as
any mathematical operations taking place on the data, (for
example as a result of a gain change, or dynamic effect
process,) may result in quantization and rounding errors.
The higher the resolution of the digital data, the smaller the
audible effect of these errors.
Q: What is dithering? Why do I need it?
A: When dropping down from e.g. 24 bit to 16 bit,
quantizing errors occur, (because 24 bit sampling involves
more samples than 16 bit, so when you reduce the bit
depth the extra samples have ‘nowhere to go.’) At high
signal levels these errors are random and not audible, but at
lower signal levels the errors correlate more closely to the
audio and become audible as distortion. Dithering effectively
‘randomises’ the truncation errors at lower levels, causing
the 'least significant bit' distortion to disappear.
Q: Can I retrofit a digital board to an analogue ISA
428 at a later date?
A: Yes, and you can do it yourself - it can easily be retro-
fitted at any time without any soldering etc, just a few
screws to undo, and one clip-connector to join to the main
PCB.
Q: How can the ISA 428 operate with the ADAT
lightpipe format at 96kHz? I thought the maximum
sample rate for this format was 48kHz?
A: Not any more. The ISA 428 supports the new 96kHz
ADAT specification, using two discrete optical ports.
Q: How many digital outputs can I use at the same
time?
A: It depends on which sample rate you choose.
At 48kHz or lower: 8 AES + 8 S/PDIF (or 2 x 8 AES) + 2 x
8 ADAT = max. 32 outputs simultaneously.
At 96kHz: 8 AES* + 8 S/PDIF (or 2 x 8 AES) + 8 ADAT =
max. 24 outputs simultaneously.
At 192kHz: 8 AES* + 8 S/PDIF (or 2 x 8 AES,) no ADAT**
= max. 16 outputs simultaneously.
* NB Both AES single-wire and split-wire configurations are
supported; split wire will obviously reduce the maximum
number of simultaneous outputs.
**At 192kHz: the ADAT ports are muted.
The digital outputs can always be fed with any mix of mic,
line, inst. inputs.
Q: When the A/D is used, are my analogue outputs
available for use?
A: Yes. The 4 line outputs can run simultaneously with all of
the ADAT, AES and S/PDIF outputs.
Q: Do I need to buy an option cable to use either of
the A/D cards?
A: Yes; ADAT optical cables are available from many
sources, and Focusrite offer their own 8-channel 9-pin to 4
phono (RCA) connectors S/PDIF cable and 9-pin to 4 XLR
connectors AES-EBU cable.
Содержание ISA 428 Pre Pack
Страница 8: ...ENGLISH 8 Signal Connections ...
