Digital Output Option; Mic Pre-Amp Input Impedance; Variable Impedance: In Depth Explanation - Focusrite TwinTrak Pro Mode D'emploi

DIGITAL OUTPUT OPTION

In addition to the analogue outputs, a high quality 24 bit, 128x over-sampled digital
output may be fitted as an extra cost option, and can operate at sample frequencies of
44.1, 48, 88.2 or 96 kHz. All of the following functions are available on the rear panel
when this option is fitted:
- This 24 bit output is S/PDIF format on an RCA phono
S/PDIF OUTPUT
connector. If 16 bit resolution is required, the receiving device should dither the 24 bit
signal to achieve 16 bit performance.
SAMPLE FREQUENCY (switch)
sample frequencies as marked on the rear panel. The left-hand switch selects between
44.1 kHz (switch in) and 48 kHz (switch out), and the right hand switch doubles the
selected frequency, providing for 88.2 and 96 kHz sample frequencies.
EXT WORDCLOCK INPUT
BNC connector, the TwinTrak Pro will attempt to synchronise to it. When the unit is
correctly locked to the external wordclock source the ADC LOCK LED will be
illuminated (see above). See Wordclock on page 10 for more details.
Fitting the card
See the separate digital option owner's manual for instructions on how to fit the ADC.

MIC PRE-AMP INPUT IMPEDANCE

A major element of the sound of a mic pre is related to the interaction between the
specific microphone being used and the type of mic pre-amp interface technology to
which it is connected. The main areas in which this interaction has an effect are the
level and frequency response of the microphone, as follows:
- Professional microphones tend to have low output impedances and so
LEVEL
more level can be achieved by selecting a higher impedance position.
FREQUENCY RESPONSE
tailored frequency responses can be further enhanced by choosing lower impedance
settings. Alternatively, choosing higher input impedance values will tend to emphasise
the high frequency response of the microphone, allowing you to get improved
ambient information and high end clarity, even from average-performance
microphones. Various microphone/pre-amp impedance combinations can be tried to
achieve the desired amount of colouration for the instrument or voice being recorded.
- Two switches give a choice of four
- If an external wordclock source is fed to the
- Microphones with defined presence peaks and
To understand how to use the impedance selection creatively it may be useful to read
the following section on how the microphone output impedance and the mic pre-amp
input impedance interact.

VARIABLE IMPEDANCE: IN DEPTH EXPLANATION

Dynamic moving coil and condenser microphones
Almost all professional dynamic and condenser microphones are designed to have a
relatively low nominal output impedance of between 150 Ω and 300 Ω when
measured at 1 kHz. Microphones are designed to have such low output impedance
because they are then less susceptible to noise pickup and they can drive long cables
without high frequency roll-off due to cable capacitance.
The side-effect of having such low output impedance is that the mic pre-amp input
impedance has a major effect on the output level of the microphone. Low pre-amp
impedance loads down the microphone output voltage, and emphasises any frequency-
related variation in microphone output impedance. Matching the mic pre-amp
resistance to the microphone output impedance (e.g. making a pre-amp input
impedance 200 Ω to match a 200 Ω microphone) still reduces the microphone output
and signal to noise ratio by 6 dB, which is undesirable.
To minimise microphone loading, and to maximise signal to noise ratio, pre-amps
have traditionally been designed to have an input impedance about ten times greater
than the average microphone, around 1.2 kΩ to 2 kΩ. (The original ISA 110 pre-amp
design followed this convention and has an input impedance of 1.4 kΩ at 1 kHz.)
Input impedance settings greater than 2 kΩ tend to make the frequency-related
variations of microphone output less significant than at low impedance settings.
Therefore high input impedance settings yield a microphone performance that is more
flat in the low and mid frequency areas and boosted in the high frequency area when
compared to low impedance settings.
Ribbon microphones
The impedance of a ribbon microphone is worthy of special mention, as this type of
microphone is affected enormously by pre-amp impedance. The ribbon impedance
within this type of microphone is incredibly low, around 0.2 Ω, and requires an output
transformer to convert the extremely low voltage it can generate into a signal capable
of being amplified by a pre-amp. The ribbon microphone output transformer requires
a ratio of around 1:30 (primary: secondary) to increase the ribbon voltage to a useful
level, and this transformer ratio also has the effect of increasing the output impedance
of the mic to around 200 Ω at 1 kHz.
ENGLISH
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