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DC signals

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DC signals

PostPosted: Fri Feb 22, 2019 9:17 am
by ef37a
https://mail.yahoo.com/neo/b/message?sM ... ckimages=0

The above link speaks of "DC coupled outputs for control purposes" This is a technology (amongst many!) that has passed me by, can anyone shed any light?

I would have thought the very last thing you would want to do is send control signals down an audio line? I am in any case not a fan of DC coupled audio? Few of us wish to record changes in barometric pressure.

Dave.

Re: DC signals

PostPosted: Fri Feb 22, 2019 9:59 am
by The Elf
Take a look at Expert Sleepers Silent Way. That takes the idea and runs with it. I reviewed it for the mag many years ago.

Re: DC signals

PostPosted: Fri Feb 22, 2019 11:17 am
by Wonks
ef37a wrote:https://mail.yahoo.com/neo/b/message?sMid=3&fid=Inbox&fidx=1&sort=date&order=down&startMid=0&filterBy=&ac=OlP3eTwe6c0gI2PV0.KFKBhdDS8-&reason=unsupported_browser&.rand=1084440976&midIndex=3&mid=AOTIVZRlgZwRXG8KaAIqsN5jPYQ&fromId=&blockimages=0

The above link speaks of "DC coupled outputs for control purposes" This is a technology (amongst many!) that has passed me by, can anyone shed any light?

Link requires you to sign into Yahoo Mail (which 99% of us can't do - and maybe it only works for you, Dave . So no idea what's you are talking about.

Re: DC signals

PostPosted: Fri Feb 22, 2019 11:40 am
by Hugh Robjohns
ef37a wrote:I would have thought the very last thing you would want to do is send control signals down an audio line? I am in any case not a fan of DC coupled audio? Few of us wish to record changes in barometric pressure.

Whether you can record barometric pressure or not will depend on the mic, but only a specially adapted omni mic capsule and impedance converter would allow it anyway. I believe there is a custom mod option with the Sennheiser MKH20 to allow an extremely extended LF response... but it's not something that's going to be a concern to any normal microphone!

But DC control signals on audio circuits aren't new -- indeed it makes a lot of sense given that there is no wanted audio signal below 20Hz (or higher), allowing relatively easy and complete separation of control and audio components.

Phantom power can be thought of as a DC control signal; Neumann have several models in which the polar pattern can be remotely control by altering the phantom power voltage, for example.

H

Re: DC signals

PostPosted: Fri Feb 22, 2019 12:09 pm
by CS70
Hugh Robjohns wrote:.
Phantom power can be thought of as a DC control signal; Neumann have several models in which the polar pattern can be remotely control by altering the phantom power voltage, for example.

Ah that’s how they do it then! I’d always wondered.

Re: DC signals

PostPosted: Sat Feb 23, 2019 9:49 am
by ef37a
Sorry about the link, don't think that has happened before?

Phantom power is a bit of a special case IMHO and of course is found only on an input and we are always cautioned to keep faders down when interrupting 48V.

A DC coupled output seems to me a somewhat dodgy thing to feed an active speaker? Then, whilst op amps are pretty much bomb proof against driving a short I am not happy about them being vulnerable to DC in the outside world? Phantom power might get connected. In general I like my "audio" to occupy the necessary bandwidth we hear, both at the low and high end. The fashion for pre amps capable of MW reception is a silly one IMHO.

I dare say Presonus have taken some clever steps to protect the outputs but why not put control signals on 3.5mm TRS? They are becoming common for MIDI now and that is effectively DC and "we" know that.

Dave.

Re: DC signals

PostPosted: Sat Feb 23, 2019 10:19 am
by Folderol
I've no experience of this sort of thing in audio stuff, but in industry it's considered bad practice to mix control, signal and power.

Re: DC signals

PostPosted: Sat Feb 23, 2019 10:41 am
by Hugh Robjohns
Combining signal, control, and power has been standard practice on plain old telephone lines for longer than any of us have been around -- and the combination has been essential to its long term success... As always, though, the devil is in the detail.

Sadly, however, I still have absolutely no idea what prompted Dave's original post, or what specific tech he's talking about as I don't have a yahoo login (and no desire to acquire one), so I can't offer any valid opinions on that...

Re: DC signals

PostPosted: Sat Feb 23, 2019 11:52 am
by ef37a
Ah! Sorry Hugh...https://www.presonus.com/products/Studi ... Interfaces

Better? Telephones. Like the electric guitar, that was the technology of the day. I hope we would do different now!

Dave.

Re: DC signals

PostPosted: Sat Feb 23, 2019 12:04 pm
by Hugh Robjohns
ef37a wrote:Ah! Sorry Hugh...

Ah ha! Thanks for the link Dave. I think there reference to being able to send control voltages is so that you can use the interface to generate CVs for synths and the like -- I've certainly heard of applicatons that use interfaces in that way.

Better? Telephones. Like the electric guitar, that was the technology of the day. I hope we would do different now!

We do, obviously, with data fibres and the like.... but the POTS system is still universally employed across the planet, and it works extremely well for its designed intentions. However, the specific point was that the system does combine power, control signalling, and audio all on the same lines at the same time, very effectively and efficiently. So it can be done safely when necessary, if engineered appropriately.

H

Re: DC signals

PostPosted: Sat Feb 23, 2019 1:05 pm
by Folderol
Hugh Robjohns wrote:[if engineered appropriately.

H
Even today, the POTS system uses quite extreme protection systems - especially for overhead lines, so I don't think that's a good comparison. Industry generally is more cost conscious, and audio? Well it's getting better but still questionable except from the very high end kit. Look how long it took most people to 'discover' that semiconductors can't handle static the way valves can.

FX: gets off hobby-horse, pats it on the bum and sends it away

Re: DC signals

PostPosted: Sat Feb 23, 2019 2:39 pm
by ef37a
Folderol wrote:
Hugh Robjohns wrote:[if engineered appropriately.

H
Even today, the POTS system uses quite extreme protection systems - especially for overhead lines, so I don't think that's a good comparison. Industry generally is more cost conscious, and audio? Well it's getting better but still questionable except from the very high end kit. Look how long it took most people to 'discover' that semiconductors can't handle static the way valves can.

FX: gets off hobby-horse, pats it on the bum and sends it away

And even longer to realize that even "audio" transistors have a bandwidth up into the short wave and the venerable 819 FET makes a bloody good Band ll pre amp! In fact it took legislation (EU?) to get basic RFI proofing fitted into kit.

Dave.

Re: DC signals

PostPosted: Sat Feb 23, 2019 3:06 pm
by wireman
Hugh Robjohns wrote:Combining signal, control, and power has been standard practice on plain old telephone lines for longer than any of us have been around.

I was going to mention that earlier but the control bit is not so obviously DC, ring is AC and line state detection is based on the state of the device on premises.

Re: DC signals

PostPosted: Sat Feb 23, 2019 4:51 pm
by Hugh Robjohns
Yes, the ringing signal is AC (17Hz if I recall correctly), but the old rotary dial system worked by interrupting a DC current loop -- hence DC signalling on the same line as the audio, and the 50V battery power.

Just sayin'... The combination can be done if necessary, even if it is expensive to engineer... :lol: