On 23/08/2010, at 11:23 AM, Rob Danielson wrote:
> What is the 66dB gain spec about in the Mix Pre manual and why did SD
> make the displayed gain settings in their units so misleading?
Hi Rob,
Sorry I've been adding a "-" to the required input figures, which probably =
hasn't been helping matters.
It's not misleading at all as they very clearly specify the reference they =
use for the measurements. The SD recorders are designed for the US film and=
tv market and use the SMPTE alignment level of -20dBFS, rather than 0dBFS.
With digital recorders you can only measure the gain in reference to the si=
gnal level recorded to digital file. This means gain can only be expressed =
correctly in relation to the dBFS reference against which it is measured. =
With a mixer you can measure gain as the difference between signal at the i=
nput and signal at the output, so gain can be expressed as an absolute dB f=
igure. This is the case with the Mix Pre.
Think about the difference in signal required to register -20dBFS and 0dBFS=
without changing the gain on the recorder. You'll obviously need a substan=
tially louder signal to increase the meter reading from -20dBFS to 0dBFS. =
So what the SD specs are saying is at full gain you need a 67.4dB input to=
the mic preamp to reach -20dBFS in the recorded file. If you increase the=
reference level to 0dBFS you need to increase the level at the input by ro=
ughly 20dB to reach a 0dBFS level. The gain of the preamp hasn't changed b=
y altering the reference level, but the level of the signal at the preamp i=
nput has. In both cases difference between the input and the reference is =
Message: 47.
Subject: 4dB, and this is the amount of gain being added by the preamp (and adc
a=
s Raimund points out).
The line input on the SD never reaches unity gain in the sense of 0dB =3D 0=
dBFS - the maximum "gain" is +18dB to -20dBFS. This translates to -2dB to 0=
dBFS meaning at maximum "gain" the line input attenuates the signal by 2dB.=
The Sound Devices 7-series are built around the -20dBFS reference, so th=
e gain readout on the display reflects the input to -20dBFS.
cheers
Paul
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