Digital Latency In Looped Effects?(Are ya there Rick?)

[Jeremy H]

Hi guys,
So after sending my first mix for mastering, the mastering engineer (who shall remain nameless - but he sponsors this forum :wink: ) calls to very courteously and professionally advise that my mix had some "serious phase issues", particularly related to a few critical elements of the mix and possibly the effect that had been assigned to those individual tracks.

The set up is an Alesis HD24 (using the internal converters for record and playback) and a big ol Ta@#$% 3500 console for mixing. I also have an outboard rack of sorts with GE, compressors...and a DIGITAL reverb unit.

I suspected that the phasing issues must be coming from the effect send return loop - so I set about to check that I didn't have a reversed lead or similar.

Firstly, I sent a 1k test tone from the console out of a master output, through a short patch lead and back into an effects return input. Raise the effects return fader and the return signal adds to the original and the VU meters rise with the effect return fader. (keep going and it feedsback!). I believe that is what it should do -unless there was a phase reversal inside this part of the console. (or the patch lead)

So then I plugged in my usual send return leads - sent the test tone to the patch bay at the outboard rack, used a short patch lead to jump the send straight back to the return at the patch bay. Raise the effect return fader - the signals combine and add - as previously and as you would expect unless there was a polarity reversal in the send return leads or the patch bay or the patch lead - all ok so far.

So I check the leads between the patch bay and the digital reverb unit - no polarity reversal there. But to be sure, I plug my send return leads directly into the back of the digital reverb unit - set up a reverb patch on the unit, but set the internal FX amount to zero - and the unit to "bypass" mode.

So now I have a test tone going through my send cable, directly into the digital reverb unit, out into my effect return cable back to my console. I raise the effect return fader....and combine the return signal with the original and, this time, it subtracts - drastically! Until I push the fader up enough to cause feedback.

Ok, the only explanation I can think of is, that even though the digital unit is not providing any effect and is in bypass mode an AD/DA conversion is happening and the out of phase is caused by the digital latency. (Or there is a reversed polarity inside the reverb unit!?)

Now the thing is, the very first time I used this digital reverb unit, I heard this 'phasey" thing happening. (On just one track - in a more complicated mix with a lot going on, it is not as obvious to me.) I investigated and by recording test tones and recording the effect return signal - and using the "track slip" function on the Alesis recorder to put the signals back in phase - I quantified this latency at 0.7 of one millisecond. (Which I don

[PeterR]

Is the reverb unit set to 100% wet?

A "phasiness" delay issue from your reverb would only occur if there was some dry signal returning with the wet signal I think?

Peter

[Jeremy H]

Yeah Peter, thats what I thought - the unit has a "Wet/Dry Mix - Internal or External" control which I set to external. Also I still get the phase issue even with the unit set in "bypass" mode - which I guess means I'm getting a 100% dry signal so not surprising I suppose.

Unless these particular algorithms still contain some "Dry" content even with the "Wet/Dry Mix" parameter set to "External"??

We're talking be!@#$%^& 2496 V Verb here ..so I guess anything is possible But we are talking about a very basic design flaw, if that is the case. And one which renders the unit useless in a send/return loop scenario.
Jeremy

[Howard Jones]

Try switching off the power to the reverb unit and see if it passes a signal when powered off - a real hardwire bypass.

If it [a] does pass a signal and [b] still has a phase problem then you can conclude that digital anything has nought to do with the issue.

BTW, what is the make and model concerned here?

[Jeremy H]

thanks for taking an interest Howard.

the unit is a Berhinger 2496 V Verb Pro - and doesn't pass signal unless it is powered.

thanks
Jeremy

[Howard Jones]

I don't have a service manual for that model and cannot check some things I otherwise would have...

Jeremy, if you can hear 0.7 of a millisecond delay on some tracks in your mixes, then you are better than most of us out here.

I would be looking elsewhere for the source of your problem... have you read the manual very, very carefully? Are you comfortable that you have understood the way this particular unit works with respect to the wet/dry mix? Are you sure that you haven't added in a delay whilst setting up the reverb parameters?

Have you tried asking be!@#$%^& for help? (I know, I know)

[rick]

my feeling was that the problem was simple (it may not be)
my thoughts were the reverb effects or whatever caused it to sound the way it does is because the device was not true stereo
it sounds like "dodgy psuedo stereo"
with such a device the mono in comes out as mono out on the left and inverted phase on the right channel ,this is a very common way to make things sound "spacial"
the problem is if you mix that inverted right channel back into the mix any way you like it will cause wierd cancelations to the mono source material making them sound "small" .
so just use the left channel of the effect and things should sound fine.
however it may be a lot more complicated like your post suggests and beringer could easily be delaying the whole box by some small increment which would make it sound a bit like it does as well

but it does sounds like a simple common wierdness in the effects mix to me .

turn the wet mix out of the box so its all dry and do your tests again , if it ends up the same i will give you a contest of who can throw a cheap reverb box the furthest.

[Jeremy H]

Thanks Howard and thanks Rick,

The more I look at the box in question - the more I think it is the "psuedo stereo" thing Rick. In this case anyway

i'll do some more investigation and get back to you.

cheers,
Jeremy

[rob]

my recollection of the topology of be!@#$%^& digital processors ( from the my days spent servicing them a few years ago ) is that the audio comes in gets converted to digital ( A to D ) goes into a processing chip and gets reconverted back to audio ( D to A ). Thus in any mode including bypass there is some small latency as a result of this conversion and processing introduced.

However, if you are dialing up a reverb that is 100% wet there really shouldn't be an issue as with most reverb programs you'd have some pre-delay set up as a parameter, say 10mS to 100mS and this will over shadow any latency. Unless of course the actual reverb program is including some straight phase inverted audio in one channel compared to the other. Check that you have the effect set to 100% wet. Bring up the reverb return in stereo and listen just to the reverb ( soloed ), now mono the left and right returns, is there a significant change in the tonality? ( indicating phase cancellation ) You need to do this with program or noise, tones will confuse the issue as any delay that is a mulitiple of half the period of the frequency will cause cancellation and a reverb program will ( should ) have all sorts of multiple delays that are different between left and right.

Very short pre-delay parameters on reverbs can cause weird phase issues

this all points to the importance of always checking mixes in mono and the desirability of a phase meter as a part of a standard metering setup.

Rob

[smash]

Just a quicky....

Is it possible that if you are using an effects send not 100% wet that some of the uneffected signal is causing phase cancelation issues with the original signal?

I always try and run my fx busses at 100% wet, and use the send level to set relative wetness, the exception being if it is an effect that I am placing accross the entire mix buss, in which case i *may* use a value of less that 100%

[Jeremy H]

OK....Firstly a big "thanks" to everyone who took the time to respond to my post. As always, everybody

[Peter Knight]

BTW gents, what are we supposed to be looking for when using a phase meter? I already check in mono so I guess I'm OK there.

[chris p]

There's an old SOS article that answers that

http://www.soundonsound.com/sos/jun00/articles/metring.htm

Stay in positive territory on the phase meter is the message.

That said, the best phase meter is your ears. Rick's latest in AT is a good example - it didn't sound right even with BOTH channels out of phase! Provided you do check the mix in mono and make sure it doesn't lose its fatness quicker than a reality show housemate, you'll be right.

Hey, any prizes for the most gratuitous reference to reality TV?

[JulienG]

That said, the best phase meter is your ears. Rick's latest in AT is a good example - it didn't sound right even with BOTH channels out of phase!

The reason for that is easy if you stop and think about it, say you top-mic a snare, the first thing you'll hear is the suck instead of the blow. That's basically what Rick was hearing. Other people probably didn't notice simply because they aren't as familiar with the room and the little change that it is can easily be overshadowed by the rooms acoustics.

In that case a phase meter wouldn't have helped because the problem was in the input of the amp, but I'm sure Rick won't forget to check the phase of his system when he redoes it next.

[chris p]

In that case a phase meter wouldn't have helped because the problem was in the input of the amp

Yeah, exactly. That's why ears are better than a meter.

[Jeremy H]

Talking about absolute phase - as in Rick's new speakers scenario....What is the "standard" correlation between a microphone at the input end and the speaker at the output end of a signal chain? I would presume that the positive pressure part of the sound wave - causing the mic diapragm to "move away" from the sound source - is supposed to translate to cause the speaker cone to "move towards" the listener?

[chris p]

Yeah Jeremy, I thinks thats right. Bear in mind that the diagphram moving away from the sound should create a positive voltage signal which, when applied (after amping and processing) to a speaker, should make the speaker cone move outwards.

[rick]

the little scientest within part 3457885...?

lets be clear about that magazine article, i have actually NOT bought those speakers although i came very close.

i have not bought them because beside making me lose whats left of my hair setting them up and getting them right, and although it was amazing that they showing me an example of absolute phase making a difference for the first time EVER, but and its a big but they just do not float my boat .

when your getting new speakers or any bit of gear really , there are all kinds of hurdles to jump through but the the most important one is the most often over looked one
" no matter what anybody else says ....do i actually like listening to these things"

the duntechs amazing as they are consistently made me wish they were just a little bit better.

those beringer effects processers we are talking about must work for hundreds of studios out there but why hasnt somebody twigged that they have issues worth posting up on the net..?
well the main reason is you have to actually let the little scientest with lose to find out whats going on
and perhaps the beriger fraternity dont usually have him at hand

so that said my expectations for the ultimate reference speaker system here at turtlerock is very high and you will not believe the speaker and amp conclusion i have come to for our main room , but think big, heavy,more tubes then the london underground and your getting close .
i will tell you all about them and post up pix when
1. they are installed and sounding great
2. i have paid the bill so they dont leave and go somewhere else to hang out :)

oh yeah, starting next week we are getting both rooms 100 % rewired using exotic audiophile grade cables to replace the perfectly fine klotz install cable and i am talking about EVERY cable in both rooms and i can thank the little scientest within for that particular folly
when will it end ...?

[JulienG]

oh yeah, starting next week we are getting both rooms 100 % rewired using exotic audiophile grade cables to replace the perfectly fine klotz install cable and i am talking about EVERY cable in both rooms and i can thank the little scientest within for that particular folly
when will it end ...?

I know you say *EVERY*, but power? digital?

[Jeremy H]

Hey Rick,..... you wouldn't happen to know where I could get some perfectly fine second hand klotz cable....would you?

[rick]

i am afraid to say we are replacing every cable , digital and power included,
the iec power leads are being sorted, tagged and tested and only the ones that spec up are being reused.
what i have found as mad as it seems is that power leads make a difference to the sound on some boxes that draw heaps of current ( amplifiers - tube stuff etc) not because the exotic audiophile cables are magic but because some of the noname molded grey iec leads hardly have ANY strands of copper in them , about 1 in 10 of my cables that i looked at are what i would describe as "weakarsed power cables" and because i could not be bothered trying to match a $2 cable to the box where you cannot hear the difference i am checking and testing the entire set.

the dig stuff is getting an upgrade because we are doing everything else so why not

and sure we always have a bunch of perfect fine cable and multicore offcuts for forum members to snappel of us
just send me an email about what you might need and give us a few bucks and out it goes

[otto ruiter]

so that said my expectations for the ultimate reference speaker system here at turtlerock is very high and you will not believe the speaker and amp conclusion i have come to for our main room , but think big, heavy,more tubes then the london underground and your getting close .

I'm hoping they're like these... http://www.lenardaudio.com/main/05_opal.html. There's something about the thought of 4 x 27" woofers driven by loads of valves that makes me all gooey...

[wez]

"gooey" is exactly how i think they would sound in the wrong room ;-)

nice woodwork though.

[Kris]

Does anyone here have any experience with Evidence Audio cables? I'm about to wire my new pedal board and have been looking around for audio cables and power cables too.

http://www.evidenceaudio.com/

[Andrew]

one thing I learned just after reading 'the litte scientist within' is never open equipment in front of anyone else who is not into diy to see how it works. It seems most people get nervous when a case is opened and do not appreciate build structure?

[rick]

err otto... me thinks you know whats going on here !
certainly for the curious i am happy to say those leonard tube amp racks in that link are sitting right infront of me as i type and wow they are something way above anything i have ever seen (including my neumann tube amps which was plan A untill i set eyes and ears on these leonards.)
they are a tube 4 way active system which means there are actually 10 mono tube kt88 power amps ,

so we are using those amps but the speaker configuration we are putting in will be very different although i dont think i can see how it could be much smaller
but certainly more suited to our room and using the best drivers for each range that we can possible find.
but we are not using horns for a bunch of reasons that make sense but are not worth discussing here

our new reference system will be a fairly... err .... "excessive" a 6 hz to 100khz response is what we are trying to acheive and with design team i have working on it the pieces of the puzzle seem to stack up .
but its a bit uncharted territory for me so lets just wait and see

[rick]

hey kris i went down the "what cable for large pedal board " path a few years ago and without question the end result was worth the effort
i ended up with george L cable ( skinny ugly looking stuff) and with it was remarkably better for my 61 strat then anything else i tried , and i was pretty amazed at how some cables made the guitar sound really muddy.

i dont know anything about the one you linked to but if you want to play it safe i recommend the george l cable

[otto ruiter]

err otto... me thinks you know whats going on here !

!!! A complete guess on my part! I just figured if it was big, and valve powered it *could* be those. Sorry if I put you on the spot, I didn't expect to be on the money. Hope it all works as well as it looks.

[jkhuri44]

I second that rick,

ive been using george Ls for everything, they are rather awesome...


Kris, you can get em custom chopped at Global Vintage on Clarence St

added to that i sit in the control room with my signal patched through the walls using mercenary/Little labs STD....you cant get much more signal mojo than that :P

(I was a bit hesitant to reply coz i didnt know what you've already tried...)

[heathen]

[quote="rick"]

our new reference system will be a fairly... err .... "excessive" a 6 hz to 100khz response is what we are trying to acheive and with design team i have working on it the pieces of the puzzle seem to stack up .
but its a bit uncharted territory for me so lets just wait and see[/quote]

Ummmmmmmmmmmmmmmmmm, wow that will be ummmm whats the word, ummmm, awesome, yeah thats it.