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Re: Sony HiMD questions

Subject: Re: Sony HiMD questions
From: "Graham Evans" grayboodle
Date: Thu Jul 13, 2006 2:51 am (PDT)
(stoatwizard wrote)

> I measured both of my NH-700s, and what this does is give you an extra
> 20dB of gain on hi-sensitivity. You pay for this with a 20dB impairment
> in overload level - the NH-700 mic input clips at -31dBu on hi and -
> 11dBu on low
> sensitivity measured at 1kHz
> <>
> from the fact that the noise floor does not increase by 20dB on low
> sensitivity although the clip level does decrease by the extra gain we
> can infer that this is probably achieved by a gain reduction in the
> input stage rather than a straight 20dB pad at the input, which is a
> nice touch by Sony.

I am relieved to hear the Lows Sens setting on the HiMDs doesn't
introduce a pad although I must admit the electronics behind the setting
now mystifies me.  If it is, instead, a "less gain" setting it should
actually produce a lower noise floor (at least as far as HiMD preamp
noise is concerned) right?

But then following the link I noticed that if recording at a "low sens"
requires a higher gain than recording the same noise at "high sens" then
the noise floor is comparitively higher.

Are these results perhaps indicative of a series of gain stages with the
Sensitivity (and the clipping) determined somewhere in the middle there?

Please excuse (and correct) any ignorance or a naive/oversimplified
understanding here but...
If the HiMDs use a digital pre-amp perhaps we are talking about:
analog signal>analog preamp (not sensitive to clipping) applying
sensitivity setting
 >digital preamp (sensitive to clipping) applying mic-recording level>
HiMd encoder

(Rob D wrote)

> The Hi-MD mic pre in Hi sens should be able to handle this level
> without distortion (although ECM mics, in general, have lower SPL
> maximums).
> It would be interesting to compare the sound files produced. You
> could recreate a known, loud-enough sound source by playing a looped
> voice recording in a speaker and pointing both lavs at the speaker 9"
> away. After you have determined that the voice playback is loud
> enough to cause distortion distortion to occur in the Hi sens
> setting at a record level setting between 11-18, make two recordings
> based on normal meter saturation*:
> A) at Hi sens setting (note record level setting- which should be
> under ~8)
> B) at Low sens (note record level setting)
> *Just below the point where the final black indicator light comes on
> in the LED meter.

My feeling is that it is the input stage clipping.  I will try and get
around to the tests you suggest asap.  It isn't a PiP problem (read on)
and the low sens setting does fix it.  I'll let you know when I have
some files for you.

(stoatwizard wrote)

> > The Hi-MD mic pre in Hi sens should be able to handle this level
> > without distortion (although ECM mics, in general, have lower SPL
> > maximums).
> It may not be the HiMD mic pre clipping. Something that is pretty
> sick about my HiMD NH700 is the gutless plug-in-power capability. The
> PIP measures an acceptable 2.3V open circuit, but when I plug my
> OKMII binaurals the votage available to the mic drops to a lousy 0.7V
> on one side and 0.4V on the other. Which is a bit crap IMHO, so I got
> to poking around to find out what is going down. The short-circuit
> current is 340uA, which isn't enough to get many electrets going. I
> have two of these MD recorders, and both measure the same. This
> implies the tail resistor is 6k8 which is waaaay too big for such a
> low voltage. The result is the electret mic is being starved for
> drive current, and there is not enough voltage across the FET for an
> output of much above 0.2Vp-p. Which is why the signal disappears in
> wind blast for example.
> Starving the mic for current and FET voltage isn't going to do
> wonders for its sensitivity, headroom, linearity or noise
> performance. The solution is cheap and easy, though a right pain in
> the field. Use a battery box - it's a 9V PP3 battery, two stereo
> sockets, a couple of 6k8 resistors and two 100uF caps. That way
> you'll get about 4V across you mic, and gain headroom and some
> sensitivity (electrets seem to go low gain at FET voltages < 1V)

I have a (9v) battery box and I experimented to see if the PiP was the
It wasn't.  I will of course include this in the tests I upload so my
subjective "loud" and "soft" voice test can be empirically verified.

thanks for the discussion... more soon
Graham E.

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