Re: HD-P2 noise measurement


Pawel Kusmierek wrote:

> Yes, but the main purpose of a portable digital recorder is to record
> from an input into digital. Who would buy it and use it as preamp only?

You wouldn't, but you can't listen to digital without following it with
something analog. And you can't measure it either, because you don't
know how much analog noise (that you're measuring) is represented by a
certain digital noise level. There is no such thing as a calibrated D/A
converter.

> The measurement combines the input (Mic) noise, which is relevant, and
> output (Line out) noise, which is irrelevant.

No, it doesn't combine them, other than to add whatever noise the D/A
converter introduces, which is surely much less than the preamp. You're
using the D/A converter as part of your test equipment so you can take
an output reading that's comparable to the input.

How would you propose to test it? And more important, how would you
express the results of your test?

Let me ask you a simpler question, and maybe the answer will help you
to understand my point. Let's say you put a -60 dBu signal into the mic
input, turned the input gain all the way up, and made a recording. You
looked at the digital file and saw that the peaks were at -4 dBFS.
What's the gain? If your answer is 56 dB, you're wroing. You don't know
what the gain is until you play that digital recording back through
something that converts it to analog, so you can measure the voltage
coming out and compute the ratio of output to input voltage.

> I think that the relationship between the input level and the digital
> level should be known for such device. I'd love to see specs like:

That would be nice, but we don't know it. And even if we did know it
for one device, it wouldn't necessarily be the same for another device.
All CD players don't play back the same CD at the same analog level,
for example.

> Am I demanding too much?

No, you're describing a measurement that depends on a certain
calibration factor that isn't universal. Life would be simpler in the
specification lane if all A/D converters had the same input sensitivity
and all D/A converters had the same output level for a given digital
level, but that isn't the way it is. "Pro" converters don't usually
vary by more than about 5 dB one way or another, but if you don't have
the same relationship between analog voltage and digital level both
going in and coming out, any measurement you make will have to be
corrected for that difference. By going on the assumption that the
recorder is a "unity gain" device - that is you get the same level out
as you put in, you can at least measure gain and noise for that
particular device and know your measurements are meaningful.

.

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