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Author Topic: Sound Card Digital Interface question  (Read 27707 times)
W0BTU
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« on: July 14, 2011, 01:26:51 PM »

    I'm wondering what the practical advantages are (if any) of commercial digital mode interfaces with their own sound card, such as the SignaLink, RIGblaster, RASCAL, etc.

    For PSK31, etc., all I have here between my computer (using Fldigi) and my Icom transceivers is a very simple homebrew interface: It uses two audio transformers and about 4 resistors. No USB or serial port connection, no adjustments, no software drivers, and no PTT connection, either (it uses the transceiver's VOX).

    What am I missing by using such a simple interface vs. one that costs ~$100? Is something going over my head here?

    I see people with these commercial units complain about (among other things):

 - My computer doesn't have a serial port, and so I can't use it.
 - I'm having trouble installing the drivers supplied by the manufacturer of the interface.
 - Why is this so complicated?
 - ... and the list goes on.


        Why do we need to use a USB or serial port to get on PSK31 or other digital modes?
        Why do I need a separate sound card?
        What's wrong with using my computer's sound card?
        What is the driver software needed for?


    I have all three of my hearing aids at maximum. Please educate me. :-)

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G0GQK
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« Reply #1 on: July 14, 2011, 01:34:30 PM »

I've been using a simple octo coupler for over 10 years with the computer soundcard with a connection to ACC1 and I've never experienced any problems. I don't think you will gain anything by buying an expensive interface unless you have a computer which may cause you connection problems as some do.

G0GQK
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W0BTU
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« Reply #2 on: July 14, 2011, 01:36:03 PM »

I don't think you will gain anything by buying an expensive interface

I have come to the same conclusion. Thank you.
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KG6AF
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« Reply #3 on: July 14, 2011, 02:59:22 PM »

Units like the SignaLink USB have three advantages:

1) By having two sound cards, you can use the sound card in your computer for system sounds.  For example, when I use DM780 with the SignaLink USB, all of the tx and rx audio goes through the external sound card, while system sounds--like DM780 announcing RSID detection--go through the computer sound card and my speakers.  There's no need to mute system sounds to keep them from going out over the air.  Of course, you can achieve the same thing by adding an extra sound card to your computer.

2) Fewer cables.  The SignaLink USB has one USB cable going to the computer and another cable going to the rig's accessory port or microphone connector.  That's it.  Fewer cables means fewer little antennas that might pick up RF and throw a monkey wrench into the works.  Maybe you've read about lots of problems with USB drivers, but I haven't; most modern OSes have built-in support for external USB sound cards.

3) I like the fact that the SignaLink has pots on the front to set tx and rx levels.  That's a lot easier than searching through the computer settings to find the right software volume control.  (There's nothing to say that you couldn't put pots on a simpler interface.)

That said, there's absolutely nothing wrong with what you're using now.  If you're happy with it, there's no need to change.
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KB6HOH
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« Reply #4 on: July 15, 2011, 10:04:09 PM »

Michael and All,

           If the Computer is dedicated strictly for Digital Comms and it works for you then that's good enough for me.
Now I have 2 WinXP Desktop and a Win7 Desktop online along with 3 Laptops and 3 Signalink USB Interfaces.
The Win7 Pc is my main system doing everything from Fldigi, to Internet Ops as well as Skype Video for Training new users and also on Echolink and Teamviewer for Remote PC access.
I do alot of MultiTasking with my system. Ditto 3 times on what KG6AF said about the Signalink. I'm very involved with Emcom.
I could not do this if I didnot have the Signalink USB Interfaces. As they say KISS it!
But do what you have to do to take care of Business!

                               73 de Steve KB6HOH
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W0BTU
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« Reply #5 on: July 15, 2011, 10:28:50 PM »

Thanks for all the replies.

I've been using a simple octo coupler ...

This kind of interests me. Anyone know where I can look at a schematic of an interface using an optocoupler?
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AD6KA
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« Reply #6 on: July 15, 2011, 11:27:46 PM »

Anyone know where I can look at a schematic of an interface using an optocoupler?
WM2U has a nicely done page about sound card interfacing:
http://www.qsl.net/wm2u/interface.html
The optocoupler based schematics are about 1/3 of the
way down on the right. There are also links to more involved
interfaces (using audio isolation transformers, etc)
GL ES 73, Ken  AD6KA
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AA4PB
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« Reply #7 on: July 16, 2011, 04:46:16 AM »


Why do we need to use a USB or serial port to get on PSK31 or other digital modes?
Only for PTT control.

Why do I need a separate sound card?
You don't but it adds some convience.

What's wrong with using my computer's sound card?
Nothing unless your sound card has a problem of some sort.

What is the driver software needed for?
It provides a virtual serial port so that the USB connection looks like a COM port that can be interfaced by legacy digital software programs. If, for example, software was written to interface directly to the SignaLink-USB then the driver wouldn't be needed. But then the software would have to have a bunch of individual interface types to select from. Legacy serial gives us a standard interface to work with.



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W0BTU
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« Reply #8 on: July 16, 2011, 10:22:20 AM »

Anyone know where I can look at a schematic of an interface using an optocoupler?
WM2U has a nicely done page about sound card interfacing:
http://www.qsl.net/wm2u/interface.html ...

Thank you.

It's interesting that the first two schematics there (both of which I have used) he says "With this configuration the VOX is used to trigger the PTT line for Transmission." Why do we need the complexity of PTT when VOX works just fine?
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AD6KA
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« Reply #9 on: July 16, 2011, 03:33:46 PM »

Quote
Why do we need the complexity of PTT when VOX works just fine?
Because sometimes it does not work fine for folks who have
RFI problems in their shack.
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W0BTU
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« Reply #10 on: September 14, 2011, 05:47:55 PM »

Quote
Why do we need the complexity of PTT when VOX works just fine?
Because sometimes it does not work fine for folks who have RFI problems in their shack.

This, I simply cannot understand. If we have RF in the shack, why would it affect the ability to use VOX? Why would RF on the audio line prevent us from using VOX, and not cause worse problems getting into the mic audio input? Am I missing something?

It's now been two months since I posted this here and on qrz.com, and that's the only reason anyone has given me for the need for PTT when operating PSK31.

All over the Internet, we find computer-to-radio interface schematics with PTT circuits. But I (and others) operate PSK31 using Fldigi, and have NO connection to the transceiver's PTT line. The audio from the sound card keys our rigs using my rig's VOX function just fine.

I have updated http://www.w0btu.com/digital_modes.html. There are more links and questions there about the need for PTT.
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KX5JT
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« Reply #11 on: September 14, 2011, 10:30:17 PM »

If VOX works for you great.  PTT works for me just fine.  There's no worrying about latency and all that jazz with PTT.  I think it's great to have both options.  There are some kit rigs out there that do not offer VOX support so they will need PTT capable software. 

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STAYVERTICAL
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« Reply #12 on: September 15, 2011, 02:44:22 AM »

I have built many homebrew interfaces, and they will work fine - BUT the problem is that your configuration of cables etc
may work fine for you but not for others with different RF environments.
I found the biggest problem for me was RF getting into cables it shouldn't, causing strange problems.
If you build a good interface, bypass and shield it properly it should be as bulletproof as a Signalink USB.
However, a poorly built one can be a perpetual source of frustration, although perhaps not in your particular shack.

The USB signalink is merely an external USB sound card with a built in VOX circuit activating a PTT relay.
The audio isolation is similar to that used in homebrew interfaces and is just two audio transformers.
It has the advantage that since the only cable going in is a USB cable, it can be effectively bypassed simply.
This is compared with the two audio cables of varying quality which are usually used in homebrew interfaces.

Also, not using the onboard soundcard for digimodes is a boon for other uses ranging from listening to music/movies or even
listening to the digitised voice alarm from programs such a Ham radio deluxe/DM780, not to mention not having system
alarms going to air.

Also, having the two soundcards allows one to use the PC as a great Audio Digital signal processor for Digimodes and CW,
greatly increasing the versatility for no extra cost.

As to using PTT compared to VOX, the main thing I find is that the same circuit used for PTT (normally using RTS) can be used
with the DTR line for CW keying. This means needing a USB to serial dongle for most modern laptops, since they dont sport
serial ports nowdays.

The main disadvantage of the USB signalink is the delay in PTT operation that will limit modes such as ALE400 or other ARQ
modes, and having to use MCW to send CW through the SIgnalink.

I use both a homebrew and a USB signalink as they both have their advantages, although I must admit the USB signalink gets
at least 95 percent of the use on digimodes.

For those who want a plug and play solution which will almost always work quickly and well the USB signalink is a good option,
but for those who want to homebrew, well that is what makes ham radio fun.

73s
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VA7CPC
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« Reply #13 on: September 15, 2011, 04:51:15 AM »

Quote
This, I simply cannot understand. If we have RF in the shack, why would it affect the ability to use VOX? Why would RF on the audio line prevent us from using VOX, and not cause worse problems getting into the mic audio input? Am I missing something?

You're missing something.  Consider yourself lucky to have a rig which supports digital VOX -- it _really_ simplifies the world.

The "digital VOX" feature -- being able to use VOX with an audio signal on "DATA IN" pins, rather than through the microphone socket -- is very recent.   My IC-706 (mkIIg) doesn't have it, my FT-817 doesn't have it.   My FT-450 _does_ have it.

So anyone with an older (more than 4 years or so) transceiver, who wants to use a high-level audio signal (>100 mV) to drive the rig, _must_ use a circuit to give PTT switching.

That means either:

. . . A transistor-switching circuit run off a serial-port pin (Rascal, and any other interface that
. . . . . . needs a serial-pot input from the computer)

or

. . . A transistor-switching cricuit run off a VOX-like detector (SignaLink, and some recent homebrew designs in QST).


RFI is a separate problem.  I once had a situation (SSB phone) in which VOX didn't work because of RFI.  After I stopped talking, the rig stayed in XMIT mode -- there was enough feedback from the transmitted signal into the audio line to keep VOX on.  That was a nasty problem, solved with several large ferrite chokes.

              Charles
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W0BTU
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« Reply #14 on: September 15, 2011, 12:55:01 PM »

You're missing something.  Consider yourself lucky to have a rig which supports digital VOX -- it _really_ simplifies the world.

The "digital VOX" feature -- being able to use VOX with an audio signal on "DATA IN" pins, rather than through the microphone socket...

I have older Icom rigs (IC-765 and IC-751A) and neither has digital VOX.

When I operate PSK31 on either rig, I have the sound card analog audio output connected to the microphone input (through an audio isolation transformer).

As far as a VOX circuit is concerned, audio is audio, whether it is voice from the mic or audio from the sound card. Both instantly put the transceiver into transmit mode. This is what people just cannot seem to grasp.

It is a very simple matter to unplug the mic and plug the sound card into the mic connector when I decide to operate PSK31 instead of SSB.

And before anyone asks, the IMD of my OTA PSK31 signal reported by other stations is as good or better than anyone else's.

I still don't see what I'm missing.
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