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A Workable Solution

water2

I suppose this is a workable solution which has some practical values. The upper plot is a mini band scope, and the lower plot is an aid for CW tuning. The audio output is band-pass filtered with the parameters adjustable by the sliders before being sent to the USB audio output device, Zoom G3.

gnuradio11

Note that the card and the device numbers may change, and now Zoom G3 is hw:1,0.

user1@UNIX [03:39:45] [~/Music] 
% aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: MID [HDA Intel MID], device 0: ALC889 Analog [ALC889 Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: MID [HDA Intel MID], device 3: ALC889 Digital [ALC889 Digital]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: Series [ZOOM G Series], device 0: USB Audio [USB Audio]
  Subdevices: 0/1
  Subdevice #0: subdevice #0
card 2: CODEC [USB Audio CODEC], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 3: HDMI [HDA ATI HDMI], device 3: HDMI 0 [HDMI 0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
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Linux Sound System

The Linux sound system is rather complicated partly due to historical reasons. It depends on how you construct you system, or which distribution you use. For example, if you wish to use a Firewire (IEEE 1394) sound device, the minimum requirement is: lilbraw1394, llibffado, libsndfile, jackd, and your application program, which you can write in one hundred lines or so in C.

Assuming you have installed ALSA in your system, you can find your sound devices and play sound as in the following manner:

aplay2

Card 0 is the device on the motherboard, Card 1 is on the display with HDMI sound capability, Card 2 and Card 3 are USB sound devices on IC-7410 and Zoom G3, respectively. Two songs are played simultaneously using two different devices, namely Card 1 and Card 3.

alsa3

This is a list of input devices. Card 1 does not appear here because it is an output only device.

user1@UNIX [10:36:01] [~/Music] 
% arecord -f cd --device=hw:2,0 IC7410.wav 
Recording WAVE 'IC7410.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Stereo
user1@UNIX [10:37:18] [~/Music] 
% aplay --device=hw:3,0 IC7410.wav
Playing WAVE 'IC7410.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Stereo

Record a sound from IC-7410 and play on Zoom G3, both via USB I/F.

user1@UNIX [10:38:21] [~/Music] 
% alsaloop --cdevice hw:2,0 --pdevice hw:3,0

This is a loopback between two devices.

slip

In this configuration the sound from IC-7410 is distributed to both the display and Zoom G3.