Strange oscillations

[Headroom]

Strange oscilllations are ruining the S/N ratio in my DM4800. I can get any fat channel to produce oscillations at a level of about -50 dBu. They start when I feed some signal into the channel with the equalizer on and continue after the signal is switched off. By tweaking the equalizer settings I can produce any tone. It happens on more than one channel simultaneously, so my DM has actually turned into a 48-tone polyphonic synth Oscillations stop when I switch off the equalizer and don't start again as long as there's no input signal. Now I know that there can't be oscillation unless there is some sort of feedback, but I can't imagine what can cause this.

I hope someone can help!

Frans

 

[Jarno]

I discovered this behaviour when The Old Forum was alive and wrote some detailed posts about it. Conclusion was: oscilation only happens, when there's complete silence on the EQ after it has had some signal present.

Workaround is to feed some low-level noise into the EQ. Any analog signal (DM's A/D converters etc) has enough of noise to prevent this happening. If your input is a DAW, you can insert a noise-generating plug-in (like dithering plug-in) to stop the oscillation. So, while the oscillation is always below -80dBFS (usually unaudible) with sensitive material, you can always add some noise (below -140dBFS) to stop it.

This is definitely a bug in DM, which should be fixed, but definitely not a show stopper, just a nuisance.

 

[Headroom]

Jarno,

Is there anything you DON'T know about the DM's?

Thanx a lot!!

Frans

 

[Jarno]

There MUST be something! And I can bet there is ...

 

[Jarno]

And for anyone interested in the topic. The next one is generated with DM-4800's EQ only. No external synths/other audio sources:
http://www.saunalahti.fi/~jvarjoti/dm/oscillation.mp3

 

[captdan]

Good gracious, Jarno! Sounds like Kraftwerke with a terrible hangover.

CaptDan

 

[webadres]

Jarno I also have this oscillation problems, I believe it has something to do with tdif connections. I use a lot of digital connetcions (firewire card/adat and 24 tdif channels) Do you have a solution, or a way to feed een low noise signal into the digital path? Thanks very much for replying

erik

 

[cmaffia]

Strange Oscillations. Sounds like a great title for a song!

 

[Headroom]

https://soundcloud.com/chrisweeks/chris-weeks-the-sadmachine

 

[cmaffia]

Lol now that's what I'm talking about!