Audacity Forum

When recording, the music pattern on the display centres symmetrically around the "0" level; i.e. the positive amplitude is of same value as the negative amplitude. However, occasionally the waveform goes straight negative (value -1), then gradually (exponentionally) the waveform returns to centre (value 0) again.
1. Why the sudden negative transition?
2. How to correct the waveform using "effect"?
PS. My theory is that my record player skips. Each skip causes a no-input to the amplifier and the input detection circuitry bias makes the display go to negative (value -1).

Are you using a dreaded, evil USB Turntable?

Koz

kozikowski wrote:Are you using a dreaded, evil USB Turntable?

Koz

No, I am using an old-fashioned turn table which does not have an USB connector. The 2 conductor red white cable connector inserts into the plug of my computer i.e. line input.
Also very recently I installed a new stylus on the record player, just to make certain that it would not skip too much on the old scratchy vinyl records.

It sounds like a hardware problem - something like a capacitor breaking down and putting a DC voltage on the audio input, which gradually (exponentially) returns to the correct level (zero DC offset).
Can you test the line in of your soundcard by connecting a different audio source to it?

stevethefiddle wrote:It sounds like a hardware problem - something like a capacitor breaking down and putting a DC voltage on the audio input, which gradually (exponentially) returns to the correct level (zero DC offset).
Can you test the line in of your soundcard by connecting a different audio source to it?

I tested the Line Input of my sound card with another source. The source was a diskette player. It does not show the abrupt negative transition. So might this be an electrical problem? or a mechanical problem with the record player ?

Can the waveform be manipulated to move it from way negative (level -1)to centre (level 0) by using the "effect" options?

<<<The 2 conductor red white cable connector inserts into the plug of my computer i.e. line input. >>>

No Phono Preamplifier in the middle? Do the transfers sound OK, not high and peaky?

<<<Can the waveform be manipulated to move it from way negative (level -1)to centre (level 0) by using the "effect" options?>>>

From your description, you'll need to fade the transition in or out according to the duration of the distortion. Do any of the disturbances occur during the performance? Can you just reduce the transition to zero with Edit > Silence.

It's really difficult to get a non-audio tool. There isn't much call for a "Pulsing Battery" effect.

One of the "add-on" tools under "Effect" is DC Bias Removal. That may help depending on how they wrote it.


Having said all that, every fiber in my being is screaming to find out what really happened. Phonographs don't just take it upon themselves to generate DC pulses. Something is broken. Phonographs also don't plug directly into computers. This is a sample of a raw turntable and then one with a phono preamplifier applied.

http://www.kozco.com/tech/riaa/riaademo.mp3

Sound on phonograph records is intentionally distorted like that. It's part of the physical process of putting sound on vinyl.

If all your music sounds OK, then there is a preamplifier hidden somewhere in your signal pathway and that's what may be in trouble.

Koz

There is one Audacity tool-based solution to this and it can be made to work perfectly. It's the sample point editor.

Load an audio track into Audacity and keep magnifying it until you can see the little dots on the waveform. Those are the digital sample points and they can be moved around with the sample edit tool which looks like the little pencil in the upper left tool panel.

You have to be careful not to take out the sound, just the gradual DC bias of your pulse or pop, so you may have to go back and forth a few times before you hit it.

Oh and you have to edit left and right separately.

Koz

hagekay wrote:I tested the Line Input of my sound card with another source. The source was a diskette player. It does not show the abrupt negative transition. So might this be an electrical problem? or a mechanical problem with the record player ?

It certainly sounds like a problem with the turntable.

As koz was saying, there should be a pre-amp between the turntable and the line input on the computer. Some turntables have this built in and offer both a "phono out" (straight from the cartridge without the pre-amp) and "line out" (after amplification and RIAA equalisation by the pre-amp).

If your turntable has a pre-amp built in then the problem may be there.

kozikowski wrote:This is a sample of a raw turntable and then one with a phono preamplifier applied.
http://www.kozco.com/tech/riaa/riaademo.mp3

That's a good demo koz - here's the same recording, but this time it starts with the raw turntable, then it is the raw turntable, but with RIAA equalisation applied from the Audacity effects, and finally with a hardware pre-amp:
http://easyspacepro.com/audacity/riaademo2.mp3

Quote: "No Phono Preamplifier in the middle? Do the transfers sound OK, not high and peaky?"

The record player is part of an stand alone old-fashioned stereo system. The Line Input of the computer soundcard gets its input via a red-white cable which is plugged into the back of the stereo. Those same jacks normally connect to the external left and right speaker.
No doubt the left and right speaker jacks come from the amp in the stereo. Thus not from the record player's pre-amp. In more words: the sound output comes from the raw record, then into its pre-amp, then into the stereo amp, then onto the speaker jackets, then via the red-white wire to the Line Input.
Attached is a picture of the waveform. It ain't distorted, saturated or clipped; and it sounds normal. The puzzle is: why the sudden negative dip and then the gradual (exponential) curve to the centre (level 0).

Ideally you would connect the pre-amp out to the line in - that is, before the power amp. A speaker output is not designed to go into a line input, for one thing, the signal level can be much higher than line level (so I hope you are keeping the volume level on the record player very low). The other problem is that the impedance (resistance) is totally wrong - a speaker output would be designed to drive a load of around 4 to 8 Ohms, whereas a line input is likely to be closer to 20,000 Ohms, and that could be part of the problem.

The picture of the waveform (very useful) shows very nicely that what is happening is that a DC voltage is appearing intermittently on the output of the record player (is the record player mono?). As I suggested before, this is most likely because a component (such as a capacitor) is breaking down. Capacitors are often used to separate signal waveforms from DC voltages - when they are in good working order, they will allow audio frequencies to pass through, but will block DC voltages (with just a little DC "leakage"). If a capacitor starts to break down, then the isolation of the DC voltage breaks down and you have a voltage where you shouldn't have. If this becomes severe, you will hear it through the speakers.

If the amplifier is correctly connected up to an 8 Ohm load (a loudspeaker), then the low resistance will help to keep the DC offset low (the signal will wiggle evenly either side of the centre), thus you can probably get away with a slightly unstable output without hearing it too much. A line input on the other hand does not load the outputs anyway near as much (by a factor of several thousand ), so the fault will show up much more.