What is a delay line?

[tolley1466]

I know its used on multiple speaker stacks/line arrays, but what is it actually for? And what is it meant to archive?

 

Thanks :P

[Kevin Ross]

In its simplest form a delay just takes a signal and delays it by X milliseconds (ms).

 

The purpose of this is to align in time multiple stacks of speakers for example at an outdoor festival you might have delay towers halfway back from the stage to provide sound to people seated further away from the main stage. If the sound left both the main and delay stacks at the same time the people further away would hear the delay stack first then sometime later (depending on distance) the sound from the main stacks.

 

The way I think of it is to slow things down. Imagine a pulse emanating from the main speakers and sweeping out throughout the crowd. You have to make the delay stacks “wait” for that signal to get near it before it sets it pulse off, kind of like a relay racer passing the baton.

 

Most DSPs allow this kind of function on their outputs and this method is very useful on smaller setups where even coverage is required or when room acoustics make things problematic

[Bobbsy]

Just to expand a bit on what Kevin said, once the sound is converted to an electrical signal and travelling through your cables, it's moving at about three quarters the speed of light, i.e. very fast. Compared to this, sound travelling through the air is pretty slow--about 1100 feet per second. Although not precise, for "back of the envelope" calculations of delays, most of us use a rounded off number of 1ms per foot. This means that, at a large event, if you have a delay stack 300 feet from the main stack, the sound from the distant stack has to be delayed by around 300ms. to be in sync with the mains. As I say, this is a rounded off figure. Your best bet is to start with this figure and then use your ears for the final adjustment...you'll find that the sound will suddenly "snap in" as you slowly adjust the delay and hit the right setting.

 

If you ever work in the theatre, there's an additional use of delays to consider. If you Google for the "Haas Effect" (sometimes called the "Precedence Effect") you'll get the background but a summary is as follows: In a situation where you're doing "subtle" reinforcement and the audience is going to hear a mix of direct sound from the actors and amplified sound from your FOH system, if you delay the amplified sound to arrive just slightly after the direct, psychoacoustics mean that the audience will be fooled into not noticing the amplified sound and thinking it all comes direct from the actor. Again, dial it in by ear--you'll be surprised how dramatic the effect is! (Bad theatre pun intended!)

 

One caution about the use of delays in either situation: if you think about it, the precise amount of delay required will be right only for a few seats. The setting of the delay time will always be a compromise. This is often less important for the "Haas Effect" since a range of delays will work okay, but for big concert stacks you have to be careful since the effect will be a slightly "muddy" sound.

 

Bob

[Simon Lewis]

And just to expand on what Bob has said (!)

 

The delay line is initially there to stop the multiple arrival times of sounds from two sets of loudspeakers (like having a digital delay set to a 100ms slap back)

 

The Haas Effect (adding slightly more delay than is "needed" to align the two sets of speakers) allows you to have the second (nearer) set of speakers up to 10dB louder than the sound from the speakers at the stage before they are judged to be as loud.

 

The Precedence Effect (or law of the first wavefront) means that we judge where the sound originates from according to where the first soundwave came from. If we delay the nearer loudspeaker so that its contribution arrives after the speakers at the stage, our brain tells us that the sound actually came from the stage, even though we might be hearing most of the sound from the closer, off axis, delay speakers.

 

In short, delay lines are incredibly useful - not just to align arrival times, but to make use of several psychoacoustic effects. Have a look at TiMAX fro example of how very sophisticated delay lines can be used.

 

Simon

[Just Some Bloke]

All wonderful answers (nicely described gentlemen!) and very very true. However, don't forget that you can also use a delay line as an effect instead of, or as well as, reverb. Many of John Lennon's vocals, for example, had a 100ms delay added to them because it simply made him sound more like a decent singer that way! - and indeed Phil Collins and others have been known to do the same. When people take the mick out of Elvis (as we're doing for one particular gag in our panto, for example) they'll usually change from the reverb they used for the previous song, to a big delay effect with some "feedback" in it (in other words, the initial singing is delayed then that delay itself is further delayed so you get multiple "echoes"). The reason this sounds like old fashioned singers is that, before digital reverb was invented, we only had rubbish reverb effects using things like springs, which worked on only a tiny bandwidth of the audio spectrum (ie. they sounded awful!) but you could use a reel-to-reel tape recorder to record a sound then play it back slightly later (by putting the record and playback heads next to each other but with the required distance to give your delay) which could give you much, much better quality. This is what we now do digitally in a DDL and can be used for singers, guitarists etc.

 

Just another use for the same thing, really.

[Rob_Beech]

Would anyone object to me creating a blueroom wiki page from this data and a few extra points I wish to make? Not right now this minute its gone 2am...

[Bobbsy]

Would anyone object to me creating a blueroom wiki page from this data and a few extra points I wish to make? Not right now this minute its gone 2am...

 

Good idea...and, as I was feeling guilty that I wasn't doing this myself (getting a bit busy down here just now), your kind offer gets me off the hook!

 

Bob

[nothingatall666]

Would anyone object to me creating a blueroom wiki page from this data and a few extra points I wish to make? Not right now this minute its gone 2am...

 

please do, it'd save a lot of digging when my young memory forgets the details about delays

[Rob_Beech]

I am getting round to it, honest!!!!

[obsoperator]

blueroom wiki page

 

Rob, do note that there is already some misinformation in this thread. The term is historical shorthand for a digital delay (as described here), and is short for "mercury delay line", see:

 

http://en.wikipedia.org/wiki/Delay_line_memory

 

So while spring-reverbs are an important part of sound history, they are not really comparable to real delay lines. It's kind of like comparing a 1940 military "throat mike" to a modern condenser mike!

[Just Some Bloke]

Rob, do note that there is already some misinformation in this thread. The term is historical shorthand for a digital delay (as described here), and is short for "mercury delay line", see:

 

http://en.wikipedia.org/wiki/Delay_line_memory

 

So while spring-reverbs are an important part of sound history, they are not really comparable to real delay lines. It's kind of like comparing a 1940 military "throat mike" to a modern condenser mike!

 

Obsop, perhaps you'd care to read my post one more time! At no point do I say that a delay line has anything whatsoever to do with a spring reverb!!!! Indeed, your simile is well chosen: I said the equivalent of "because 1940 throat mikes were so crap we ended up with the modern condenser mike instead". Just to help you along, I've copied my exact wording here:

 

All wonderful answers (nicely described gentlemen!) and very very true. However, don't forget that you can also use a delay line as an effect instead of, or as well as, reverb. Many of John Lennon's vocals, for example, had a 100ms delay added to them because it simply made him sound more like a decent singer that way! - and indeed Phil Collins and others have been known to do the same. When people take the mick out of Elvis (as we're doing for one particular gag in our panto, for example) they'll usually change from the reverb they used for the previous song, to a big delay effect with some "feedback" in it (in other words, the initial singing is delayed then that delay itself is further delayed so you get multiple "echoes"). The reason this sounds like old fashioned singers is that, before digital reverb was invented, we only had rubbish reverb effects using things like springs, which worked on only a tiny bandwidth of the audio spectrum (ie. they sounded awful!) but you could use a reel-to-reel tape recorder to record a sound then play it back slightly later (by putting the record and playback heads next to each other but with the required distance to give your delay) which could give you much, much better quality. This is what we now do digitally in a DDL and can be used for singers, guitarists etc.

 

Just another use for the same thing, really.

 

Perhaps you could point out where the misinformation is in that? :** laughs out loud **:

 

I've also looked at the wiki page you link to which gives a Mercury Delay Line as an early example of a delay line, but certainly doesn't suggest that "delay line" is short for "Mercury Delay Line". Mercury is one particular type of delay, digital is another.

 

As someone who grew up trying to find better and better ways of recording vocals on a budget because the only decent reverb units cost a grand each, I can assure you I went through the whole gamut of spring reverbs, tape delays, Wem Copycats (!), and a whole load of other stuff before finally Yamaha released the SPX90 and we all breathed a sigh of relief! - now at least a few of us could club together and share one! :** laughs out loud **:

[paulears]

I'm confused? I'm not aware of any audio delays every being carried out using mercury or other metal delay lines? As I remember my history these types of delay had very narrow bandwidths and weren't much use in a musical context - real reverb rooms, spring delays and tape echo being the only things I every used. I too had a WEM copycat, and used tape echo from a Ferrograph, before buying the Great Briish Spring Reverb in it's tube.

[obsoperator]

Just Some Bloke, by not "quoting" your post I hoped to make my comment less personal than you may have chosen to take it. The phrase I objected to was in fact part of your re-quote:

before digital reverb was invented, we only had rubbish reverb effects using things like springs, which worked on only a tiny bandwidth of the audio spectrum (ie. they sounded awful!) but you could use a reel-to-reel tape recorder to record a sound then play it back slightly later (by putting the record and playback heads next to each other but with the required distance to give your delay) which could give you much, much better quality

As a history lesson, this is particularly misleading to younger readers who've never used a "stock" three-head reel-to-reel recorder, or helped set up a back line with a Roland Space Echo or a WEM Copicat. Tape delay devices were widely used and respectable in audio quality (not "rubbish") if the tape loop was replaced when needed. I haven't found a brand name, but there is some evidence that early magnetic drums were used for analog audio signals without the "wearing-out" problem of tape loops. I set up a Space Echo in 1983, but I don't know how long they remained in use.

 

While very special applications were done with two separate reel-to-reel machines, it was always possible to produce a two-choice delay (i.e. 7 1/2 IPS and 15 IPS... tape speed) with a "three-headed" professional recorder as taken out of the box. No machining, hardware, or additional budget monies required.

 

Although I don't like to rely on Wikipeda too much, I think I chose the third-best page for my previous post, so I'd also like to offer:

 

Delay Line (without the "memory" word), and

Delay (audio effect)

 

Having explained why I judged your post to include misleading content for a Wiki article, I must acknowledge that I was wrong in saying that mercury delay lines were sold for live sound audio delay. I cannot find a reference to support that. I was misled by the frequent use of the terms "acoustic" or "analog" to describe mercury delay lines. I now believe this was because mercury and piezoelectric transducers were used together because their impedance at acoustic and ultrasonic frequencies was very close.

 

Although I have not found any reference to the specific derivation of the word "line", I found a first-person UK engineer's reminiscences of why the word "tank" was often used instead of "tube" to describe the mercury container. That may be important because SoundBlaster used the term "tank engine" to label their digital delay process.

 

I do apologize for my own contribution of bad content.

[paulears]

I cannot find a fault with JSBs comment - spring reverbs were everyday items - and I believe that Hammond, the organ manufacturer introduced spring reveb before the second world war. This was before tape based echo systems were introduced into the music field. Spring reverb was and still is pretty horrible, although from the seventies onwards transducers got as lot better - however, the metalic 'flavour' is still there today.

 

I think JSBs explanation of tape based systems does explain it - certainly in enough detail to allow somebody to do more research. Roland Space Echo was a pretty good alternative to the Copycat, but not being British, meant WEM was more popular here in the UK.

[dbuckley]

Frankly, I'm glad we live in a age when all that junk is behind us. The only spring reverb I have is the one in my 70s vintage Yamaha organ, which is a constant reminder to me how awful these technologies are.

 

I've done the B77 delay, had the copycats and space echos, and even at one time aspired to own a Great British Spring, which was obscenely expensive (about 300 quid if memory serves). Had a relatively expensive Dynacord tape delay.

 

Thank your local deity for digital technology. The worst, cheapest delay box on the market today will thrash the pants off a copycat every day of the week. Heck I gladly swapped the copycats for analogue delays units, Electro Harmonix memory men. They were actually of a lesser quality than a Copycat, but had a lower maintenance requirement. 'Tis true, the copycats served me well for yonks, but they got consigned to the loft by the memory men, and forgotten about, and when we moved to New Zealand, they got left in the loft in the UK. I still come across tubes of tape loops...

 

Actually, I lied in para one - I have a WEM Halle Cat rotting in a shed, which hasn't been powered on for well over a decade, which does tape echos and has a spring line in it. 'Tis in the same shed as a Teac four track reel-to-reel that cost a fortune...

 

Got a giggle out of the note on the John Lennon vocal - a much better example is Noddy Holder, who in his heyday was never heard without his fixed delay, provided by a (I'm sure at the time hideously expensive) Eventide delay unit. When the song ends and the vocalist goes into announcement mode, I always mute the thickeners, but Noddy's soundman never did: Noddy always had the thickened voice.