Curiously, I am currently contemplating designing a small digital mixer for
location recording (not specifically to MD, but entirely usable with
it). My day job is DSP design (both hardware and software) for recording
studio equipment (www.cedaraudio.com); my hobbies are performing and
recording classical music (oh, and furniture making...). It recently
occurred to me that a small digital mixer is quite feasible as a DIY
project with careful design. Other complicated bits (such as FPGA designs
and firmware) can be distributed in binary form.
<< 3. Adjust levels *before* the A/D to prevent overload while maximizing
dynamic
> range. Most modern MD recorders have fixed gain *berfore* the A/D; not a
> good thing! >>
It is technically correct that to maximise ADC performance you should do
the gain control before the ADC; however, with modern ADC chips you can get
extremely good performance (up to about 100dB dynamic range, depending on
details) with this analogue gain control being pretty crude (I am planning
3 or 4 settings spanning approximately a 50dB range). I agree that most
small MD recorders do this to keep the cost down with little regard to
audio quality, and probably don't even have the crude analogue gain
switching that I suggest.
The performance I'm aiming for will not be the same as that obtained with
truly professional ADCs costing megabucks, but the DSP will be second to
none and I expect the unit overall to be good enough for producing
commercial CD releases. The unit will be portable but not battery powered.
I'd expect the parts cost to be of the order of $400 to $500 for a 1-off.
My project essentially comprises 8 XLR mic-line inputs, each with
switchable gain and phantom power. The resulting audio is passed to a DSP
which has independent level and pan controls for each channel, a master
level control, and a parametric EQ on the output. It would be possible to
have EQ on individual channels but it would make the control surface either
expensive, or fiddly to use, and I'd rarely use it in my application
anyway. As well as the digital master out (dithered/truncated as necessary)
there is a separate analogue monitor out which will drive headphones, but
which is really designed to drive a pair of active monitor speakers
directly. This monitor out normally follows the main mix, but also allows
solo and PFL functions which don't interfere with the main mix (which is
what gets recorded). There will be decent level meters on the monitor out.
The control surface will probably comprise 9 faders (8 inputs plus a
master), plus an LCD display, soft keys and opto encoder to set other
parameters.
The make-or-break of this project probably depends on whether I can get a
hobbyist electronics magazine to publish the design. The purpose of this
posting is too see if there is significant interest in this forum that I
can use as leverage with potential publishers, so let me have your comments!
<< While working on a unrelated
>project a few years ago, I used a 16 character x 2 line LCD as a high
>resoltion
>analog bar-graph meter with software controlled ballistics, as well as for
>its normal
>text display function.>>
Good idea, and one I have used too!
Christopher Hicks
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