Equalisation for PC mic input/line input

Discussion in 'DIY Computers' started by David Peters (UK), Mar 12, 2006.

  1. QUESTION:
    Do the PC mic and line inputs use the same equalisation (on the
    frequency spectrum)?


    BACKGROUND:
    I have some noisy voice tapes from an old analogue dictation machine.
    I'm reading the recordings from the "ear" socket of my dictation
    machine into my PC and then coverting the recordings to MP3.

    Later I will get some software to clean up the noise on the MP3s.

    I didn't expect it but my PC allows me to set a decent recording
    level whether I record through the mic input or the line input.

    Until I clean up the sound, the noise from the original recording
    makes it hard to tell if I'm getting a better result from the mic
    input or the line input.

    I was wondering if there was a different equalisation used by the PC
    for the mic and line inputs. If so then I would make sure I used


    ANOTHER QUESTION:
    What is the input level at which the mic and line inputs are rated?
    I had thought mic inputs were about 3 or 3 mV and line inputs were
    200 mV.

    -------

    NOTE:
    My PC motherboard chipset is VIA KT266A + VT8235.
    A PC reporting utility says it detects a VT8233/A AC97 Enhanced Audio
    Controller.
     
    David Peters (UK), Mar 12, 2006
    #1
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  2. David Peters  (UK)

    Mike Walsh Guest

    There is not equalization with line input, i.e. it uses flat frequency response. I am not sure about microphone input; the biggest difference seems to be they operate at a lower signal level.
    You should use the line input with the headphone output, as both operate at relatively high signal level with flat frequency response.
    Back in the days of analog recording 0 db was 1 volt. Since this was analog the 0 db level could be and was often exceed. With the advent of digital CDs the 0 db level became the maximum level, which can not be exceeded because of the digital format, and is supposed to be 2 volts. Since these are maximum levels the average will much lower.
    Microphone levels are lower and vary widely.
     
    Mike Walsh, Mar 12, 2006
    #2
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  3. Microphone inputs are also flat, i.e. with no equalisation. As you say, they
    operate at a much lower level, typically a few millivolts.
    Not quite. Firstly, a dB is a relative level, not an absolute, so without
    stating the reference, a figure of "xdB" is meaningless. Originally, 0dB was
    referenced to a power of 1mW into a load of 600 ohms, and was referred to as
    0dBm. Later, the same voltage level, but unloaded, that is, without
    reference to a 600 ohm load became 0dBu (that is, unloaded) Note that the
    voltage level is the same in both cases (0.775v, or 1mW into 600 ohm) There
    was a strange semi-standard evolved of referring to 1V rather than 0.775v
    and that was 0dBv.

    Digital outputs are referred to maximum digital output (when all the bits
    are 1) and that is called 0dBFS (0dB Full Scale). It has NO analogue
    equivalent, as analogue can keep getting bigger without limit, digital can't
    get any bigger than when all the bits are 1. In Digital-Analogue conversion,
    a number of different conversion levels have become more-or-less standard.
    The EBU (European Broadcasting Union) have defined 0dBFS digital to mean
    +18dBu analogue after conversion. The USA prefers that 0dBFS = +24dBu
    because that provides 20dB headroom above 0VU. A few dissidents prefer
    +25dBu as that's 1dB better than +24...........

    CD players have evolved a standard output of 2v analogue for 0dBFS, but as
    far as I'm aware, there is no official standard for this.

    S.
     
    Serge Auckland, Mar 12, 2006
    #3
  4. David Peters  (UK)

    Kevin Seal Guest

    Interesting.
    Can you let me have a reference to the technical paper for that.
    Cheers,
     
    Kevin Seal, Mar 12, 2006
    #4
  5. EBU R68-2000. I'm emailing you a copy directly.

    S.
     
    Serge Auckland, Mar 12, 2006
    #5
  6. David Peters  (UK)

    Kevin Seal Guest

    Received, thanks.

    With 0dBFS as =18dBU, that would mean OVU (+4dBU) would be -14dBFS. Most
    people I know line-up their Pro Tools rigs for -18dBFS for 0VU hence
    OdBFS is going to be +22dBU.
    Isn't it a lovely world!
     
    Kevin Seal, Mar 12, 2006
    #6
  7. Standard are great, that's why we have so many of them!

    S.
     
    Serge Auckland, Mar 12, 2006
    #7
  8. David Peters  (UK)

    kony Guest


    The short answer is that for best results you should:

    - use a quality tape deck with line-out, not the earphone
    jack of a dictation system

    - use the line-in on a fair quality sound card, not
    integrated motherboard audio with a really cheap codec.
     
    kony, Mar 12, 2006
    #8
  9. David Peters  (UK)

    Rich Wilson Guest

    Decibels, to me, seem to be overused, particularly with digital audio. And
    particularly because silence is negative infinity decibels, which isn't a
    lot of good if you're writing a computer program that can only cope with
    real numbers. What's wrong with plain old 0% to 100%?!
    (Rhetorical question, don't feel obliged to answer...)
     
    Rich Wilson, Mar 13, 2006
    #9

  10. I can't say I understand all of what you write but the parts I do
    understand are very useful to me. Thank you for posting.

    Are there any web sites or documents which explain this sort of thing
    for a beginner: rigorously but not going too fast.
     
    David Peters (UK), Mar 13, 2006
    #10
  11. Lots snipped>
    I don't know of any specific websites that explain all the ins and outs of
    analogue and digital audio. I've learned all this during my professional
    life in audio. There used to be a great magazine called Studio Sound, which
    had technical articles explaining the basics in rigorous but understandable
    form. Sadly SS has been extinct for several years, but you may find copies
    in larger public libraries.

    You may also want to look at Jim Lesurf's web sites -
    Electronics http://www.st-and.ac.uk/~www_pa/Scots_Guide/intro/electron.htm
    Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html

    He's got a lot of useful information, and what's more, it's correct!

    Canford Audio have useful information in their catalogue, you may want to
    contact them and see if they'll put you on the mailing list for the
    catalogue. www.canford.co.uk

    S.
     
    Serge Auckland, Mar 13, 2006
    #11
  12. David Peters  (UK)

    Kevin Seal Guest

    Are you trying to put us out of a job? :)
     
    Kevin Seal, Mar 13, 2006
    #12
  13. Not much danger of that, if he's such a bad programmer! :)

    BTW, what's wrong with 0-100% is that our hearing is logarithmic, so
    deciBels give a much better idea of how things sound. A 10dB increase
    in SPL sounds twice as loud, but takes ten times the power.
     
    Stewart Pinkerton, Mar 14, 2006
    #13
  14. It's quite interesting to look at levels off Freeview. I lined up the
    workshop receiver to read PPM 4 on a rare occasion when there was a test
    card and line up tone available. And as expected TV progs peak to no more
    than PPM 6. But some of the radio ones wrap the PPMs round the end stops.
    ;-)

    My best Freeview receiver out of several is a Sony VTX-D800U and when I
    changed it from the previous freebie Sagem which kept crashing I
    immediately noticed the audio level was low. Switch the set from the same
    channel on analogue to Freeview via a SCART and the difference was too
    much. Correspondence with Sony showed that they thought the TV
    broadcasters would peak to 0dBFS on FreeView instead of using the normal
    EBU line up of peak being -10 dBFS.
     
    Dave Plowman (News), Mar 14, 2006
    #14
  15. David Peters  (UK)

    don Guest

    dbm is as stated a reference of two like power values to a 1mW reference
    however the impedence does not need to be 600 ohms, it can be any value of
    ohms as long as both power values are based on the same impedence

    dbu is not unloaded but db(micro) it is as above but with a reference of
    1microWatt it is not actually a u but the greek character mu

    dbv would be a ratio based on two voltage levels and a reference of 1 volt

    power db caclulations are 10 log Pout/Pin
    Voltage db calculations are 20 log Vout/Vin

    dbFS is "decibels full scale". It is an abbreviation for decibel amplitude
    levels in digital systems which have a maximum available level (like PCM
    encoding). 0 dBFS is assigned to the maximum possible level. There is still
    the potential for ambiguity, since some use the RMS value of a full-scale
    square wave for 0 dBFS, and some use a sine wave.

    this is treated the same as voltag calculations because it is based on the
    signal to noise ratio.
     
    don, Mar 18, 2006
    #15
  16. Don,

    Do you have a reference for this statement? In 34 years in Pro-Audio I have
    never heard it expressed in this way, always referred to 1mW into 600 ohms.

    I have no diea what the above is referring to: dBFS has nothing to do with
    signal-to-noise, it is just how many dB below full-scale.

    S.
     
    Serge Auckland, Mar 18, 2006
    #16
  17. Since there is no simple way to measure power independently of
    knowledge of the circuit impedance (nor any real need to do so in most
    cases), the measuring device is simply an ac voltmeter capable of
    accurately measing ac volts (or milli or micro volts) at the frequencies
    concerned and such (relative) 'power' calibration in dbs on a voltage
    scale need to at least mention a reference impedance if absolute power
    levels are of any importance or at the very least what the actual
    voltage is for the chosen 0db level so that a correction factor can be
    applied when used with impedances (notional or actual) other than what
    the meter was origionally calibrated for.

    The lower the impedance, the lower the voltage reference for 1mW 0db
    level. The correction factor for a meter calibrated in db for a 600 ohm
    impedance would be plus 9db when used on a 75 ohm circuit if absolute
    power levels need to be known.

    If nothing else, knowing the reference impedance to which a level
    measuring meter is calibrated will inform you of the 1mW voltage
    calibration point. Of course, such meters can simply be referenced to a
    voltage, usually V or mV or Micro V, without the word 'impedance' being
    invoked. Either way, it _is_ important that you can identify the actual
    voltage reference point, either directly or indirectly implied by the
    intended circuit impedance it's calibrated for.
     
    Johnny B Good, Mar 18, 2006
    #17
  18. Thanks to Don and Glenn. I've learnt something. Looking through my old
    college texts books, I can see you're right. What comes of a narrow
    upbringing.

    S.
     
    Serge Auckland, Mar 18, 2006
    #18
  19. This puzzled me.

    The first quote (from don, not Don) is the opening part of the DBFS
    entry in Wikipedia - see http://en.wikipedia.org/wiki/DBFS. I think it
    is correct at least up to the final sentence about ambiguity. Then it
    becomes at least ambiguous itself.

    The actual ambiguity seem to be whether, when a waveform is said to
    have amplitude x dBFS, you mean the peak amplitude of the waveform or
    its RMS amplitude. Thus I think the fundamental ambiguity is not as
    stated in the Wikipedia article about whether you use a sine or square
    wave as reference.

    Like Don (not don) I always assumed with dBFS you implicitly meant the
    peak value of the waveform because of the nature of its representation
    in a system having a waveform-independent overload level of 0 dBFS.

    I had to think about this a bit when doing some FFTs (which usually work
    in power/energy terms) on quantized signals. Maybe some people are more
    comfortable to think of waveforms in power or energy terms however they
    are represented, even when power or energy is probably no longer relevant.
     
    John Phillips, Mar 19, 2006
    #19
  20. The wave-shape doesn't matter when talking about digital signals. 0dBFS is
    reached when any part of the waveform sets "all the bits to 1"
    This can be the crest of a sine-wave, the tip of a sawtooth or the flat top
    of a square-wave. If you have a meter that indicates dBFS, with a true-peak
    characteristic, you will get the same indication whatever the waveform.
    However, if you have a conventional rms reading analogue meter, driven from
    a D-A converter, then the waveform will affect the indication, just as it
    will for analogue waveforms that *all have the same peak value* The
    commonly-used EBU standard of +18dBu=0dBFS is only valid for sine waves.

    As an aside, in radio, digital metering is still done on conventional BBC
    style PPMs, which under-read by anything between 1-4dB depending on the
    programme content.(some will say even up to 7dB) I and others have tried
    persuading radio stations to use a true-peak meter, even if it is calibrated
    with the familiar BBC 1-7 scale. The universal reaction was that the signal
    was too quiet, and everyone prefered to go back to a meter they were
    familiar with, even if it didn't tell the truth, and rely on the 10dB
    headroom between the +8dBu UK peak operating level and the +18dBu maximum to
    accomodate any unseen peaks. US practice is even less precise as they still
    use VU meters and rely on the 20dB headroom between 0VU (+4dBu) and their
    +24dBu=0dBFS.

    S.
     
    Serge Auckland, Mar 19, 2006
    #20
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