Captured audio has a slight hum...

[Savant]

Hey folks, I'm on the grand ride of learning a new capture process. Right now I'm doing captures via my Canon ZR70MC using the A->D passthrough. After a false start (not realising that you don't hear audio during a capture) I'm capuring fine.

The problem I have is that I get a slight 'hum' in the background audio. Now I also got this when I was capturing to my ATI AIW, and I chalked it up to the fact that there is so much interference inside a PC that the card was absorbing it.

Now with a DV capture I figured I wouldn't get that since there is no audio connection to the PC in a DV capture. Unfortunately I was wrong.

I have done some testing. I noted that the hum is NOT present when I connect my input device (a VCR) directly to my TV. I've also tried a second VCR and am getting the same result. I've changed the audio cables as well, although I may get a new pair for the heck of it since these are older cables. (but they do have shielding)


So, I was thinking.... Instead of having to mess around with post capture audio tweaking, I'm curious if anyone has any other ideas?

I was considering using a tape deck I have as an audio passthrough device. It's an old deck, but a high quality one. (double deck Technics with Dolby B & Dolby C noise reduction filters) I'm wondering if running the audio through the deck and putting it through a Dolby B filter would help at all... I've never used it for this purpose, but I do want to minimize this hum as much as possible. I also wonder if using these filters would delay the audio to any extent that would throw it out of sync...

Of course if anyone else has tried to deal with hum I would be interested in knowing how you dealt with it.

Regards,

Savant

[Roundabout]

The problem I have is that I get a slight 'hum' in the background audio. Now I also got this when I was capturing to my ATI AIW, and I chalked it up to the fact that there is so much interference inside a PC that the card was absorbing it.

Were you using the same source (vcr, whatever) when you got this hum? Does it sound like the same frequency as what you're hearing with DV capture?

I was considering using a tape deck I have as an audio passthrough device. It's an old deck, but a high quality one. (double deck Technics with Dolby B & Dolby C noise reduction filters) I'm wondering if running the audio through the deck and putting it through a Dolby B filter would help at all... I've never used it for this purpose, but I do want to minimize this hum as much as possible.

AFAIK, the deck will not perform "pass-through" filtering of audio. It is designed to apply Dolby processing to the signal that goes to the tape as it's being recorded, and the only way you would be able to hear the effect is if the deck has a Tape Monitor function, which is only possible in decks with three heads (separate record, playback, and erase heads). Only higher end decks had this design, and no double decks that I have seen have this design (though I haven't seen all of them on the market). Also, Dolby is designed to cut tape hiss (high frequency noise) and your problem is low frequency noise.

Hum can come from a variety of sources, the most common being ground loop hum. You could even try something as simple as reversing the AC plug if it's not polarized. Try plugging the VCR into a different outlet than the PC, or the same one if it's on a different circuit. Keep the length of the wires you are using as short as possible.

You could try running a separate ground wire from the VCR chassis to the camera chassis, then to the PC. Since there are so many different things that can cause hum, you might be better off to use a low pass filter in Sound Forge or similar program after getting it on your hard drive.

[Savant]

Were you using the same source (vcr, whatever) when you got this hum? Does it sound like the same frequency as what you're hearing with DV capture?

Yeah it's very close to what I was getting before.

I was considering using a tape deck I have as an audio passthrough device. It's an old deck, but a high quality one. (double deck Technics with Dolby B & Dolby C noise reduction filters) I'm wondering if running the audio through the deck and putting it through a Dolby B filter would help at all... I've never used it for this purpose, but I do want to minimize this hum as much as possible.

AFAIK, the deck will not perform "pass-through" filtering of audio. It is designed to apply Dolby processing to the signal that goes to the tape as it's being recorded, and the only way you would be able to hear the effect is if the deck has a Tape Monitor function, which is only possible in decks with three heads (separate record, playback, and erase heads). Only higher end decks had this design

Yeah it supports it. It was a special editing deck I picked up years ago, and one of the two decks does have three heads. It does chome/metal, auto-reverse, syncro-edits, independant recording levels, Dolby B/C as well as 2x recording. A sweet deck in its time. I can just rec/pause the deck and I get the output with recording level and dolby filters applied. I tried it and it did apply Dolby passthough filtering to the audio, (I could hear the difference) but it didn't impact the hum to a significant extent.

Hum can come from a variety of sources, the most common being ground loop hum. ... You could try running a separate ground wire from the VCR chassis to the camera chassis, then to the PC.

Yeah, I was thinking about that, but unfortunately it's tricky to implement it. While it's relatively easy to run a ground between devices, the impedance of the ground wire must be significantly lower than the audio cable ground impedance, even at frequencies above the audio range, or you run into a whole other batch of problems.

You got me thinking though, so I picked a ground loop isolator instead. Worked like a charm! I was surprized at how clean the audio came out.

Frankly I should have thought of this months ago. While I normally think of ground loop isolation in car audio applications, I had neglected to consider it for home audio since it usually isn't an issue. Thanks for the indirect suggestion.

Regards,

Savant