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Stereo AD Converters

Discussion in 'Converters / Interfaces' started by audiokid, Mar 17, 2011.

  1. audiokid

    audiokid Staff

    We all know what it sounds like when we take two channels and pan them hard left and right and delay one track slightly.
    If you are recording something in stereo and the converters are not tight does it cause a similar effect?

    As an example: If we are recording a choir in stereo using two matched high end mics but running them through mid level converters with inconsistent clocking between each channel, is this why things don't quite seem tight? What starts to happen to the imaging and clarity?
    I am understanding things clearer, I'm really starting to get it here. Converter quality is even more important than I thought. I also see how one could appear fat and one more transparent.

    It gets more interesting to me.

    There is only so much a manufacturer can put into a product before it become clear how good it really is or if it is right for a two track system. If we are investing in the larger channel converters at a bargain price, the ones with multiple pre-amps included in the product, I see why a very stable power is critical to keep things true. Price factor and what we are getting in these multiple channel combo's makes me question things even more and helps me understand what the important factors arr for high end stereo tracking.

    When you start compounding track count its obvious why this becomes a mud bath. Now the question is, what is acceptable and wow, it is really starting to make sense to me now.
     
  2. Boswell

    Boswell Moderator Distinguished Member

    I know of no audio A-D products that do not use an A-D converter per channel all timed from the same sampling clock. This will mean that there is no delay (at audio timescales) between the channels in a device.

    Differences in performance between models are due to a multitude of things from the design of the audio driver and clock circuits, crosstalk minimisation, the power supply and power distribution and decoupling to the quality of the converter chips themseves.

    It's probably not appreciated in the practical audio world what really happened when chip designers made advances from the early audio A-D converters that used successive-approximation techniques. These are the types that went into the early digital audio products and, along with the companion D-A converters, were largely responsible for the "harsh" sound that characterised the first available CDs and CD players.

    Since the advent of "over-sampling" devices, a lot of the early problems have been reduced, but not without causing trouble elsewhere. Over-sampling ADCs and DACs need high-quality clocks locked to a multiple of the audio sampling rate, which means in practice using phase-locked loops to generate a clock at 64, 128 or even 256 times the basic 44.1KHz (or 48KHz etc for non-CD units), so attention focuses on the quality of the clock multipliers used. A rock-solid 44.1KHz sampling clock can be completely degraded by sloppy chip design in the multiplier circuits or by poor implementation of PCB layout and power decoupling in this area. It's the attention to this sort of thing in addition to specifying the better-quality parts that marks out good designs (products) from poor ones.

    Multi-channel (i.e. more than 2) A-D converters usually impart the same sonic characteristics on to all their tracks. If one thinks of the sound of a cheap condenser microphone, it may be possible to bury any unpleasantness from it in a mix of tracks where the others are recorded with better mics, but a project studio that multi-tracks everything using the one cheap mic risks multiplying up the audio deficiencies and producing a mix that offends the ear. Spread this example out from serial tracking to parallel operation by taking a low-cost multi-channel A-D converter unit, and it becomes clear why some designs have a sonic signature that is audible all the way through to the 2-track mix. Products that do not suffer from effects such as this are usually the higher-cost ones that have had the necessary attention applied to the detail of the design.
     
  3. Big K

    Big K Well-Known Member

    From the technical pov it is not possible to get 1 channel in an 8-channel Pre-amp offsynch, other then by design and on purpose.
    If you lock 2 separate but identical Pre-amps to one clock the difference in timing between the 2 is not audible, shouldn't even exist, unless something's
    gone haywire or you use real crappy equipment from the dark ages ( in Alsihad some Plugs caused single sided delay due to a bad or non-existing latency compensation, though).

    I worked with all kinds of good and bad, old and new converters. I never saw anything like that. The digital signal send off to the recorder or PC, card is multiplexed ( optical & wire). If there
    is a fault in the mux chip the whole thing goes wild or some channels are missing. The mux technology like in MADI or ADAT link is quite old, safe and explored. It it is no techical challenge and no budget question, anymore.

    The Sound of AD/DA converters is often depending on quality an prize. The synch capabilty is technically on the same problem-level like the rubber feet of its case, prolly less.
     
  4. JoeH

    JoeH Well-Known Member

    Yeah, I think it's mostly a non-issue at this point. I've never encountered any problems like that. As long as you're working from one master clock (again, inside the converter box), you should be fine.

    The only sync issues I ever worry about is if we're locking a variety of machines together for biggg projects; like DAWs, Video playback, digital consoles, etc. But I rarely have to deal with that; I'm just not that kind of (big) production company.

    And THOSE guys (big NYC or LA studios) have "people" who handle it. ;-)
     
  5. audiokid

    audiokid Staff

    Well just when you think you've got lol. I'm obviously not be explaining this correctly and understanding the part in converters that effects what I think is being effected. I'm an Auditory learner so its always been challenging, processing technical info from reading. It takes me longer to get it this way, but I do eventually as I go over and over things. My hearing is great and probably why I love working with sound and choose it as a profession. I'm going to post something once I get permission. I'm hoping you will all help me understand this better.
     
  6. audiokid

    audiokid Staff

    Lavry Engineering gave permission to post this in whole. It's what got me wondering about stereo imaging and how the clocking or ? could be effecting this like delay does. I have somewhat cross referenced this to non constant latency and wondered if this is one reason to invest in high quality converters with very good power supplies and dump firewire. The difference between my FF800 and the AD10 is pretty convincing. I found quoted below very interesting.

     
  7. Big K

    Big K Well-Known Member

    A little longish...
    Is this an older statement?

    Good microphone technique and good converters can spare you the mentioned problems.
    I have never considered any USB/FW devices acceptable for studio work, although it is better, now, then it was.
    Runtime, crosstalk and phasing problems before the acoustic signal hits the mics is not a converter problem and
    converters that give you problems like this, you better give a kick.

    I flawlessly record and play back several tracks at once at 0.7 millisecs and then there still is direkt listening, too.
    Btw, I consider low latency processing well as a property of a good converter ( and driver)... one property.
    As a last resort I recommend Little Labs® IBP Phase Alignment Tool Plug-In | Universal Audio or the hardware version of it.
    When doing multiple mic recordings I use a 70 cent Knackfrosch to record its sharp click for the mic/room runtimes, just in case I needed it later..
     
  8. audiokid

    audiokid Staff

    I suppose its a few years back now.
    Remember, I'm thinking these are pretty sweet for a remote rig including the 2-bus out of my mastering chain. It has the USB there as an option but I would use AES EBU personally but I'm sure others that don't have this luxury can still get happening.

    It kicks the FF800 out of the arena for certain as a 2-bus AD and pre.
     
  9. Big K

    Big K Well-Known Member

    AES/EBU is the way to go.... and can transport the higher sampling rates, too, compared to (ADAT format) opticals, anyway.
     
  10. audiokid

    audiokid Staff

    Does this include BNC?

    I'm assuming you are including the other digital interconnect options other than firewire itself, yes?
     
  11. Big K

    Big K Well-Known Member

    I would differentiate between ADAT, COAX MADI, AES/EBU, optical digital audio and USB, FW, etc.
    The latter are computer interface links. RME uses its own FW protocol to the PC cards, though.

    Of all digital audio connections the ADAT is the weakest as to samplingrates.
    AES/EBU is great, but means a lot of plugs and cable ( multicores).
    MADI is a comparatively cheap single coax connection for digital audio (64 channels over 100 m ( optical for 2000 m) , MIDI and RS 232.

    There are a range of boxes for connecting and converting ADAT and AES/EBU signals to and from MADI.
    For all the possibilities work through RME MADI info pages. It is all well described there from the guys that make 'em boxes.
    :)

    Now, I'd like to know, if you need to cover large distances in your studio. If it all sits in one rack or side by side, MADI might be an expensive option, after all, because you either need all I/O units to be MADI or you need an additional converter box to connect non-MADI Mic-pres, etc.
     
  12. audiokid

    audiokid Staff

    As it is, I am only going 3 meters
     
  13. audiokid

    audiokid Staff

    But I may use one converter in the acoustic room instead of running all that cable.

    I'm looking at two systems. I really have the stereo one complete. I love these Blacks and am interested in the AD11 now. They are compact and sound awesome. In my other thread, I am going to upgrade to a new 16 IO . I'm done with FW now. Its was a starting point for me while I gathered my hybrid system.
     

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