Is separate boxing for video switching necessary?

Rod, the chap who maintains the extremely useful Chopping List Web site (on movie censorship), asks:

While reading your informative A/V blog yet again, I was reminded of a question which you may be able to help me with. I have done some web searches but have not found any comprehensive info.The question relates to video switching. I currently use a Marantz AV9000 HT pre-amp as a processor. It has video switching for various formats, including composite, S-video and component. I’ve learnt that video switching may reduce audio performance, due to those circuits running at “higher frequencies”.

Ah hah, here is where I get to be heretical again. If the equipment manufacturer is not entirely incompetent, it matters not a whit that the A/V processor or receiver has video switching in the same box.

What damage can the video signal do to the audio signal? The transit of the signal could generate high frequency fields that in turn induce currents in the audio circuits. Solution: shield these sections of the circuits from each other. Could there be interference back through the common power supply? Sure. There certainly is (except where the switching is passive). As there is if the boxes are completely separate with their own power supplies. The important question is the level. Totally insignificant. Not measurable, not audible.

Marantz is most certainly not totally incompetent.

The video circuits do indeed run at higher frequencies. The bandwidth of a PAL or NTSC composite video signal is around 5MHz or a bit more (it’s considerably higher for high definition video). I haven’t checked out the power vs frequency spectrum of such signals, but let us assume that it is roughly even (ie. it’s very wide bandwidth pink noise), then the fact that it is high frequency is good. A video signal runs from virtually DC up to that 5-ish megahertz, so only a tiny proportion of it is in the audio bandwidth. Even if you take the audiobandwidth to extend to 100kHz, that still means that only 2% of the low voltage (1 volt p-p) low current video signal could map over onto the audio circuits.

In any case, there are other high frequency signals within an A/V processor — necessarily. The incoming digital audio signal, the DSPs, the DACs all run at greater than 1MHz. Once again, this isn’t a matter of concern, just careful design.

This entry was posted in Equipment, Video. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *