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Re: Re: Digital Distortion / Meters

Subject: Re: Re: Digital Distortion / Meters
From: Klas Strandberg <>
Date: Mon, 23 Jun 2003 22:09:04 +0200
Also analog meters are reading after the mic amp. If the signal is
distorted, you can't see it on the meters.


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.
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Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01


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