Also analog meters are reading after the mic amp. If the signal is
distorted, you can't see it on the meters.
Klas.
At 15:49 2003-06-23 -0400, you wrote:
>evertveldhuis wrote:
>> Walt,
>>
>> VU meters (analog needles) are average readings, but LED's on digital
>> equipment are always peak readings (unless specifically stated
>> otherwise on the meter or in the manual).
>>
>> The problem with digital meters is they read AFTER the preamp and
>> AFTER the ADC.
>> So if the signal is already too hot for the mic (high SPL) or too hot
>> for the preamp, it is already distorted BEFORE the AD converting, so
>> any digital level indication does not mean the signal is undistorted.
>> It just means the energy of the signal is at a certain level. It
>> tells nothing about the possible clipping of a signal.
>> You have to monitor / listen to the signal with good headphones to
>> judge wether a recording has clipping or not.
>> Most of the times it is analog clipping in or before the preamp
>> section I am afraid.
>
>Certainly the analog clipping is also a problem. Particularly with
>consumer gear. I don't have much problem with the mic, except for sudden
>close impulses into the Telinga. I'm sure some of those the mic overloads.
>
>All that the digital reading coming from the digital stream means is
>that it won't show readings over the clip level. It still shows what's
>going on when you are set anywhere near right. If it's showing you
>getting up to 0db, you are probably clipping.
>
>If the digital meters were giving you actual peaks, they would show you
>the correct pad particularly for the A/D, even if they are after
>everything. I think part of the problem is that a lot of the digital
>meters are set up to simulate the readings of analog meters, they
>average according to the standards set for analog meters. They also rise
>a decay based on the ballistics of analog meters.
>
>Unless they have a hold function they have to average, we cannot take in
>44,000 readings a second in a meaningful way even if the meter could
>display them. In actuality the meter is ultimately limited by the
>display refresh time, which is much slower. And our eyes are even
>slower. 30 frames a second is flicker free for us. Yet in 1/30th of a
>second there could be quite a few wavetops to clip. And clipping even
>just a few is noticeable.
>
>Do not trust digital meters to be giving true peaks even in the digital
>stream. I know the portadisc records much higher levels than it's peak
>reading would indicate. In some cases the difference may be enough to
>clip the A/D with a 10dB pad in the indication. Though I know most such
>situations and use a larger pad.
>
>So, with the Portadisc, the meter will not be giving you a reading
>that's direct off the digital stream, it's a average even for the peaks
>off that stream. I think the margin display does do better, but it's
>hard to watch that. Two numbers (for the two channels) constantly
>changing. (again they have to be averages) It would be nice if that had
>a longer hold. It's numbers agree with what I find after digital
>transfer into my computer. Or very close. So, it appears to be what you
>describe, calculated from the digital stream. I do check it once in a
>while to get a feel for the sharper impulses. Ideally the Portadisc's
>level display would include the margin numbers in the same display. I'm
>betting that would help a lot.
>
>It boils down to you have only the level indicator to help you. And
>experience in what happens with certain indications. Setting it without
>reference to the meter is sure to create problems. Use the meter, but
>use it intelligently.
>
>Your other choice is to carry a accurate sound meter with a good peak
>reading. Then develop a table for each mic as to what to set the gain.
>Not very practical. Though as a informal check it can help.
>
>And, yes, monitoring with headphones is a must. Though it's a different
>path through the recorder than the recording. That will help to catch
>some of the front end problems. Though I find that those do make it
>through into the recording, so are reflected in the level reading.
>
>Walt
>
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Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
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