[RTTY] mmtty sound card alignment

[Joe Subich, W4TV]

> I’ve tried to find out why things worked like this but with no luck.
> It seems unlikely that the sound card would be so far off, or that
> it wouldn’t run with the standard 11025hz, but there is a look hidden
> in there.

If you are using Vista or Windows 7, the operating system does not
allow individual applications which share the sound card to set the
sound card's sample rate.  Instead the operating system's hardware
abstraction layer (HAL) will *resample* the data stream (interpolate
data values) to provide audio samples to the application at the rate
the application requires.

When the sound card's default format (Windows Control Panel, Sound,
Recording Devices, <device>, Advanced) is 48000 and an application
requests a sample rate that is not an integral divisor of the default
sample rate the clock rate will not be exact - resulting in differences
from transmit to receive.

The default format for most "motherboard" or add-in cards (PCI/PCI-E)
is set by their custom drivers (supplied by the manufacturer) and is
generally 44,100 Hz (CD Quality).  However, the default format for
external (USB) sound cards is set to 48,000 Hz (DVD Quality) by the
Microsoft driver built into he operating system.

There are two solutions:

1) Set the clock rate in MMTTY (and MMVARI) to 12,000 - this removes
    the offset (48000 Hz Receive, 44100 Hz transmit) between the sample
    rates.

2) Reset the Windows "Default format" to 44100 Hz (CD Quality).  This
    also removes the offset.

Either solution will work if your only sound card digital program
is MMTTY.  If you use other software - particularly if you use
multiple programs at the same time or switch between programs,
changing the MMTTY/MMVARI sample rate to 12000 may be a better idea
since there are some programs that use fixed 48000 Hz sample rates.

73,

    ... Joe, W4TV


On 1/6/2012 8:43 AM, Mark n2qt wrote:
> During the RTTY Roundup practice session I found EVERYONE was responding to
> me about 40-50 hz
> off.  I use FSK on a k3 and have all the setting ‘right’ no afc, tone
> frequencies are correct; the rig has
> the correct tones/offsets etc.   I run SO2R so it was easy to verify my
> rx/tx tones were not aligned on
> the one rig.
>
> I am using a new motherboard sound card, a Via HD something or other.  I’ve
> never had to calibrate
> a sound card but figured it was that or play with the frequency settings in
> mmtty.
>
> One of the suggested settings besides the familiar 11025hz, was 12000hz.
> Changing to it aligned
> my transmit and receive tones perfectly so I didn’t need to do anything
> else.
>
> I’ve tried to find out why things worked like this but with no luck.  It
> seems unlikely that the sound card
> would be so far off, or that it wouldn’t run with the standard 11025hz, but
> there is a look hidden in there.
>
> So here’s one more thing to check out if you notice a lot of off freq
> callers (or if you get asked for
> repeats when answering others).
>
> Mark n2qt
> (hopefully on freq for RU!)
>
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