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Computer <-> phone-line USB audio interface

Discussion in 'Electronic Design' started by Christian Brunschen, Apr 28, 2005.

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  1. Greetings everyone,

    First of all, let me admit that I am very much a software kind of person,
    and I expect that everybody here knows the old saying,

    'The three most dangerous things in the world are a programmer with a
    soldering iron, a hardware type with a program patch, and a user with an

    which puts me among the first in that short list. So, fair warning has
    been given. Nevertheless, I shall brave my own foolishness and request
    your enlightened insight and, possibly, assistance.

    I have a Mac mini which I want to set up as the nerve centre of my home -
    including as a dial-in server, fax and phone answering machine. The Mac
    mini has a built-in modem, so the dialing-in and faxing part is covered.
    However, the modem does not have voice functionality; so I need something
    else to connect the phone line to the Mac.

    I have, in my 'research' (and I'm using this term very loosely, since it
    includes mainly googling for possibly useful phrases), come across two
    companies that make just that kind of device - but alas, only available as
    part of a product including software:


    Both of these offer a combination offer of a hardware device that connects
    the phone line to a Mac's USB port. However, both of these companies only
    offer the device in question in a bundle with their software - which I
    don't really want, being a software type (as I mentioned) and reasonably
    happy to attack that part of the issue myself. So, I'm thinking about
    building such a device myself.

    I've looked a bit further (in other words, more googling) and come up with
    an outline for what I need:

    1) a DAA (Data Access Arrangement), with a phone hybrid (2-to-4-wire
    converter), to interface the 4-wire audio that I want, to the 2-wire
    phone line (one that works specifically in the UK, where I happen to
    live at this time, so one that conforms to CTR21)

    2) a USB audio CODEC, to convert from the audio to USB

    [ Note: I am making the assumption that this device will always be used on
    parallel with a modem, or other device which can go off-hook, and thus
    doesn't need to concern itself with such matters. This should simplify
    the device, because it can simply provide a straight-through audio path,
    without having to worry about other control functions. ]

    Looking for DAAs, I found the following:

    - Mitel MH88422 - seems to do what I want
    - Infineon DAA2000 - a two-chip chipset
    - Xecom XE0092 - only certified for use in the US, not UK
    - Xecom 103X - certified for UK use, but lacks a 4-to-2 wire converter
    - Cermetek CH1837 and CH1840, both approved for US but not UK

    Of these, the Mitem MH88422 looks nice and simple and straightforward, and
    does exactly what I need it do do, without including more features that I
    *don't* need.

    On the USB audio codec, there are options such as:

    - Texas Instruments PCM2900 - USB audio CODEC (stereo, but oh well)
    - Philips UDA1325 - similar, but available as SDIP as well as for
    - CMedia CM108 - similar to TI PCM2900

    Of these, the PCM2900 looks like a nice, modern chip, but is only
    available in surface-mount packaging, and though I have soldered the odd
    thing, I'm not sure how great that is for me in the longer run. Oh well.

    Now, assuming I'd use something like a combination of the Mitel MH88422
    and a TI PCM2900, I'd obviously also need a few other bits and bobs - such
    as power. The USB bus helpfully provides some, which I was planning on
    using (rather than stealing it from the phone line, which would be
    another, though less good, option).

    It is at this point that I reach the boundaries of my limited knowledge
    and skills in this area, and would appreciate insight, comments, even
    assistance from anyone who might be inclined to offer such.

    Or, to phrase it more clearly: 'What do I do now? Help! Please!' Anything
    you could add would be much appreciated.

    Best wishes,

    // Christian Brunschen
  2. Nico Coesel

    Nico Coesel Guest

    How about buying a voice modem for USB?
  3. That's one solution, but with several drawbacks:

    1) it means I'd have two modems, one of which would be reduntant (the one
    that's built into the Mac), which would feel wrong, like a waste of

    2) it would have to be a controller-based modem, not a host-controlled
    one, and most USB modems don't specify which they are, so it's all too
    easy to end up with a modem that's the wrong kind. Host-controlled modems
    need specific drivers, rather than presenting themselves as essentially a
    serial device; remember, I want to connect it to a Mac, not a Windows
    macine, and most modem manufacturers don't make such bespoke drivers
    available for the Mac.

    3) a voice modem would require special software for handling audio,
    whereas something that presents itself as a standard USB audio device is
    immediately supported by the OS and thus by all the audio recording,
    editing and playback software already out there, and would thus be much
    easier to interface with software-wise.

    4) what would be the fun in that?

    Best wishes,

    // Christian Brunschen
  4. Tom

    Tom Guest

    I've done something simmilar with a PC, except I'm using the built-in audio
    "line in" and "line out" to play and record from the telephone line. The modem
    detects the ring and takes the phone off the hook. The only hardware that I
    needed is a simple phone line audio transformer and some resistors &

    If your Mac doesn't have regular audio inputs and outputs, or if you insist on
    using USB, then this will be a lot more difficult.

  5. Great. Do you still have a list of parts, or a schematic, or any other
    info really, from that project?
    Well, I have an existing Audio USB interface (a Griffin iMic,
    <>) that I could use. But
    with parts like the TI PCM2900, which contain all the hard- and software
    for Audio <-> USB conversion, it should be fairly simple and
    straightforward to connect such a component to something like your phone
    line audio transformer (or perhasp the Mitel MH88422 line interface
    circuit), and have a working whole so to speak - with USB providing power
    for it all, as necessary.

    It is precisely that last step - putting it all together - that I had
    hoped to find some assistance with in this forum.

    Thank you for your comments,
    Best wishes,

    // Christian
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