Subject:Flat-lining in Forge -- unexplained clipping

Posted by: jxbjxbjxb

Date:1/4/2004 7:23:53 AM

I have posted online 3 screenshots from Sound Forge to help describe a weird problem. They are at:

http://www.skylarkweb.com/jbridge/

I recorded a violinst the other day using an Audio Technica stereo mic and a Sony PCM-M1 Dat Walkman. The sound quality is generally good and I was careful not to record at high levels. I transferred the audio into my PC using a SPDF connection to a M-Audio Delta 66 card. As you can see from the waveforms posted above, the recorded audio was well within the boundaries of good levels, nowhere near the 0 dB level.

But there is clipping! I cannot understand it. You can clearly see flat tops to many of the waves, and I just can't understand why this would be. It sounds distorted, though not terribly, but it is not as clean as I would have expected! I listened to the DAT through headphones as well, and the clipping is evident there also, suggesting my sound card is not introducing the problem.

I am still skeptical that my problem was high levels. I had the DAT record level pretty darn low. If it was a clipping issue with recording, wouldn't the Forge waveformes show that by displaying waves past the 0dB line? The dB in Forge should be the same as the dB on the meters of my DAT when I recorded, since I used SPDF for the Forge input, these levels should be identical, no? But the clipping indicated occurs at several dB lower than where you'd expect clipping during a recording. What could cause clipping at the -14dB line??

This happened once before to me, using a Sennheiser shotgun mic and a more expensive DAT field recorder, and I never understood it. The levels were never too hot, but the clipping is there anyway. Since I've experienced this twice now using totally different equipment, I can only assume it is a problem that others have noticed from time to time also.

Any pointers or explanations are helpful, as I'm really confused.

Jaxon

Subject:RE: Flat-lining in Forge -- unexplained clipping

Reply by: Rednroll

Date:1/4/2004 10:08:27 AM

There is one major point of distortion possibility you are overlooking. That is within the Mic itself. A mic only has so much throw of the diaphram, before the diaphram bottoms out. In condenser mic's, along with there being a diaphram that can bottom out, there is also a pre-amp within the mic. Either of these stages can introduce distortion. Try placing a PAD on the condenser mic if there is one available. You might also try moving the mic back a little away from the sound source. You're on the right track of narrowing this down.

There's one self proclaimed "industry professional" who's been bashing me regularly on the Vegas forum, who him and his "professional" coleagues had this same type of problem recording a vocalist. Those professionals narrowed it down to the singers voice........which is physically impossible due to laws of inertia. I suppose if any of these professionals had gone to one of my classes and learned the basics of how a dynamic, condenser, and ribbon mic work they might have been able to figure out the limitations of the mic itself and wouldn't still be scratching their heads.

Subject:RE: Flat-lining in Forge -- unexplained clipping

Reply by: ClassicalWW

Date:1/4/2004 12:54:06 PM

This most likely represents overload of the mic preamp in your DAT recorder. Since the record level control follows the preamp, it won't matter how conservatively you set the level because the clipping has already occured ahead of the control.
Solutions: (1) use a microphone with lower output level. The mic preamps on many portable recorders are designed for dynamic microphones, and some condenser mics can easily overload them; (2) if your microphone or DAT recorder includes a pad to reduce the level, activate it; (3) insert an in-line pad between the microphone and the DAT recorder mic input; or, (4) feed the microphone to an external mic preamp or mixer, one which includes a "trim" control that will adjust the amount of gain in the first stage amplifier, then send the preamp or mixer output to the line input of your DAT recorder.

Subject:RE: Flat-lining in Forge -- unexplained clipping

Reply by: rraud

Date:1/4/2004 5:24:28 PM

Record vu meters are just a reference and can't always be trusted...
Us old school guys learned this early in the ol' analoge days, especially with synth's, what looks good... an't nessesarily good..

Subject:RE: Flat-lining in Forge -- unexplained clipping

Reply by: Rednroll

Date:1/4/2004 10:15:36 PM

ClassicalWW,
I haven't looked at a schematic for a DAT player, but through my other hardware design experience I have to disagree with you, unless you can point me to some information that says otherwise. The trim controls are usually part of the pre-amp circuit with the gain governed by 2 resistors, where the total gain of the preamp is -R2/R1, then follow by another pre-amp with unity gain where R2 is set equal to R1 to put the signal back in phase, but with no additional gain added. R2 is usually a variable resistor on the first preamp, and is your trim control. Thus if you want more gain you increase the value of R2 and if you want attenuation you lower the value of R2 so that it is less than R1.

RRaud,
You might have to change your thinking when it comes to digital peak meters. I know those analog machines had VU meters, and you are definately correct in this scenario. That's because VU meters don't respond quick enough for signals that have a high transient response. So if you recorded a Highhat, snare, or crash cymbal and the meter was reading 0dB VU, then you would definately be getting distortion, due to the transients going well above this and the meter not reacting quick enough. If JXB's DAT player has VU meters than you are totally correct in this assumption. If it has peak meters, than something peaking at -14dB should not have any distortion.

Subject:RE: Flat-lining in Forge -- unexplained clipping

Reply by: ClassicalWW

Date:1/5/2004 9:02:30 AM

Hi, Red.
Sony doesn't appear to have any tech information about the PCM-M1 DAT recorder on their website. If it's anything like the Tascam DA-P1, however, the mic preamp is a fixed-gain stage with the record level control functioning as a simple attenuator. There is no trim control for the preamp, and the gain is set at +60 dB, which is way too much for use with high-output condenser mics. I have seen this type of first-stage clipping even with the DA-P1's -20 dB pad engaged, thus an external pad or a preamp or mixer WITH variable gain is sometimes necessary.

I agree with you about the metering -- I've never seen a peak-level meter that reads too low, although sometimes they are set to read too high.