5hz Frequency shifter to prevent feedback

[Starstruck]

Hi Folks,

 

A friend gave me an old article from wireless world (1973 !!!) that presented a design for feedback suppression. It did it using a 5 hz frequency shifter and claims to be able to increase the stability margin of a sound system by around 8dB.

 

Has anybody used something similair ? I noticed that a few conference equipment manufacturers have them built into thier products ?

 

Would I be wasting my time building one to try it ?

 

Cheers

 

Starstruck

[TonyMitchell]

Kevin

 

I wouldn't bother building something specifically for that, why not simply test the gain before feedback difference in your own working environment using an FX processor (SPX or similar)

 

Tony

[Rob_Beech]

There are a few things to point out, on each side of the argument.

 

For a start, 5Hz at 63Hz I a significant change in pitch. Noticeable to say the least in musical program material.

However, not much feedback goes on down there and 5Hz at 4000Hz isn't a big deal.

 

Processing is going to be a killer, getting it to sound natural and do the trick may prove to be difficult.

 

 

And finally,

 

WHY? Where is your gain before feedback problem? There are certainly many more methods I'd put first.

[paulears]

I built one of those at about that time - but it was everyday electronics's version I built. Wireless World was a bit too much for me then. I remember it working really well on a single microphone. Memory tells me it was a Shure 545 before they started using XLRs plugged into a WEM PA100 with 2 4 x 12" columns. Quality stuff. As only singing went through it in those days, I can't comment on what it did to the sound, but I certainly remember getting more volume, even though the thing did hum like a good 'un!

[Starstruck]

WHY? Where is your gain before feedback problem? There are certainly many more methods I'd put first.

 

 

Its more out of curiosity than need.

 

When I set up my live systems I normally ring them out to get rid of potential feedback and if the layout is bad (and can't be fixed) I sometimes put a sabine feedback suppressor in as well and I don't normally have problems (apart from idiots pointing mics at speakers).

 

As I say its out of curiosity and I woundered if anybody actually uses this method. The article was written in 1973 so I guessed that if it actually really worked everybody would be using it by now but I don't see many people doing it.

 

Yes at lower frequencies a shift of 5hz is quite big but could you actually notice it ?? Don't know personally.

 

Ive got a microverb 4 that has a pitch shifter I wonder if that can go as low as a 5hz shift ?? Hmm time to dig the manual out.

 

Kev

 

A concurrent post has been automatically merged from this point on.

 

I built one of those at about that time - but it was everyday electronics's version I built. Wireless World was a bit too much for me then. I remember it working really well on a single microphone. Memory tells me it was a Shure 545 before they started using XLRs plugged into a WEM PA100 with 2 4 x 12" columns. Quality stuff. As only singing went through it in those days, I can't comment on what it did to the sound, but I certainly remember getting more volume, even though the thing did hum like a good 'un!

 

If I built one then I would be looking to use it as an insert so It would only be one mic.

 

The design uses some very old technology in Integrated circuits (it was 1973 !) and I was thinking about upgrading the design. The hum can be dealt with.

 

Thanks

Kev

[Shez]

I recall reading a discussion many moons ago about these, possibly on the sound on sound forum. The comment I remember in particular was that when feedback did kick off, it did it in a far more ferocious and unique sounding manor than normal and tended to result in fried HF drivers. I think a rapid upward frequency sweep rather than the steady tone we all know and love is how it sounded. Be careful...

[Starstruck]

I recall reading a discussion many moons ago about these, possibly on the sound on sound forum. The comment I remember in particular was that when feedback did kick off, it did it in a far more ferocious and unique sounding manor than normal and tended to result in fried HF drivers. I think a rapid upward frequency sweep rather than the steady tone we all know and love is how it sounded. Be careful...

 

The article does talk about the fact that each time the signal fed back then it would do it 5hz higher giving you a frequency sweep but each frequency would be of lower energy than the last one so would die in energy very rapidly. But interesting point never the less.

 

Kev

[KevinS]

There was at least one commercial device using this technique at about that time. I came across one once but I forget who made it.

I only used it for speech and not for music so can't comment on the effect it had on musical pitch but it did seem to do the job.

 

K

[Brian]

... but I forget who made it...

Surrey Electronics?

[dbuckley]

I built one of these which I still have, though it hasn't been used for a many a year.

 

It does exactly what it says on the can. For speech, anyway; I woudn't recommend it with music.

 

As noted above, Surrey Electronics built the commercial version, and a couple of years ago someone patented the idea, the concept of "prior art" apparently not being applied, as is so common with patents these days.

 

Worth noting that the design used analogue frequency shifting, so there wasn't the combination of delays one gets using digital techniques.

[djandydee]

I had a horrible experience with an install that went wrong - A mono speaker cluster was incorrectly placed behind the minister who preached in a sincere wisper! and liked to wander out into the congregation. He used a lapel radio mic and the place was an 18thC hard wall reverberant nightmare. I tried all the usual EQ tricks but could simply get no level before feedback. I tried a feedback destroyer but it just pulled out everything from 200hz to about 6K. Another engineer lent me his shifter but as described above, the feedback just started whistling tunes which were distracting and only achieved a couple of dB of level.

 

The only cure to this was a re-work with new speakers left and right with the horns mounted to give coverage to the congregation and allow a shaded triangle where he could preach from.

[Stan Hope-Streeter]

<br />Hi Folks,<br /><br />A friend gave me an old article from wireless world (1973 !!!) that presented a design for feedback suppression. It did it using a 5 hz frequency shifter and claims to be able to increase the stability margin of a sound system by around 8dB.<br /><br />Has anybody used something similair ? I noticed that a few conference equipment manufacturers have them built into thier products ?<br /><br />Would I be wasting my time building one to try it ?<br /><br />Cheers<br /><br />Starstruck<br />

<br /><br /><br />

 

Trevor Brook of Surrey Electronics manufactured these to the Wireless World design by Hartley Jones (who went on to work for Neve). Trevor manufactured a lot of useful gadgets, mainly for the broadcast industry and usually in distinctive blue boxes. The Shifter had some minor updates over the years, newer lower noise IC's etc.

 

Having used them in the past I offer the following comments:

 

They work OK in a reverberant room, but are not much use outdoors where there isn't a dense reverberant field.

 

If you can hear your own voice too well via the PA then the effect is quite disconcerting.

 

The same facility was available on the original Eventide Harmonizers but they used digital instead of analogue processing.

 

Unless you want to do it as a learning exercise and perhaps adapt it for sound effects, I wouldn't bother building this. You can get better results using a high quality parametric or graphic EQ, or an adaptive EQ such as a Sabine FBX - none of which was available in 1973. Shifters sometimes come up for internet auction, where they sell for under £50; this is probably less than it would cost to build one.

 

HTH.

[Starstruck]

Thanks for all the replies folks.

 

Looks like you have had some mixed results ! Im glad I asked the question.

 

If im looking for something to do I might still build one just to play with (ive got access to the parts for free) but based on what you guys are saying I don't think it will be any use to me for the type of gigs I do.

 

Interesting concept thou.

 

Thanks folks

 

Kev

[mervaka]

I'm suprised noone's mentioned phase shifting and/or inverting yet?

[Gareth Owen]

A year or so I experimented with using a pitch shifter on the vocal stage foldback ONLY for a theatre show. The experiment was firstly to determine how far I could shift the vocals on stage before the cast a) noticed and b) stopped singing in tune; and secondly to acertain if there was any dramatic gain before feedback improvment.

 

Pitch Shifter in question was the Digidesign Pitch Shifter running on a Digidesign Venue. Cast mics were DPA 4063s, stage foldback was d&b E3s.

 

Assuming that foldback is rolled off at 100Hz, and referencing the OPs original spec of 5Hz I discovered no detremental effects to the cast on stage (ie they neither noticed or started to sing out of tune) but I did acheive approximatly 3db more headroom. I would however concur with Shez that when it did start to go it went more abruptly than you would normally expect.

 

Conclusion? You can certainly gain more level with minimal side effects providing its only another couple of db your after.