Amplitude Modulation - Single Sideband Radios in V+

Using the latest features of VCOMM+, AM SSB (Amplitude Modulated Single Sideband) is now significantly more realistic than previously possible.  Most military HF radios use AM SSB for the modulation scheme.  Tuning these radios to receive a signal when the exact transmitting frequency is not known requires some practice since the incoming signal is garbled as you tune across the incoming frequency.  This is caused by the nature of the modulation, but manifests itself as the equivalent of pitch shifting the incoming signal as well as a number of other anomalies.

The new AM SSB feature in VCOMM+ coupled with SimPhonics V+ software enables the user to create realistic SSB DIS/HLA radios.  This is made possible using DirectX and Windows XP, allowing the incoming audio stream to be pitch shifted in real-time and mixed with a sine wave along with other effects before the stream is sent to the SMx or other sound card channel.

When the receiver is tuned within the transmitter bandwidth, the model then calculates the frequency difference between the transmitter and the receiver.  If both frequencies are the same (difference = 0), then the signal will be heard in its original form (un-modulated).  If there is a frequency difference, the received signal will be modulated by a sine wave whose frequency is equal to the difference between the transmitter and receiver tuned frequencies.

Enabling AM SSB simulation

AM SSB effects are available in V+ build numbers 210 or greater.  (VCOMM+ software builds 100 and higher)  The AM SSB effects are only available on radios that were created with a system type of SYS_GENERIC. For both the transmitting and receiving radios, the modulation must be set as follows:

Major Modulation:

• MM_AMPLITUDE (enum value = 1)

Detail Modulation (any of the following):

• AM_LSB (enum value = 6),

• or AM_SSB_FULL (enum value = 7),

• or AM_SSB_REDUC (enum value = 8),

• or AM_USB (enum value = 9)

Example

Assume a transmitting radio is set to:

• Tuned Frequency = 2.6 MHz

• Bandwidth = 1 KHz

• Major Modulation = 1 (MM_AMPLITUDE)

• Detail Modulation = 7 (AM_SSB_FULL)

and a similar receiving radio is set to:

• Tuned Frequency = 2.6002 MHz

• Bandwidth = 1 KHz

• Major Modulation = 1 (MM_AMPLITUDE)

• Detail Modulation = 7 (AM_SSB_FULL)

The resulting received audio signal will be modulated by a 200Hz sine wave resulting in a pitch-shifted, garbled signal. The received audio will also be slightly attenuated due to the fact that the tuned frequency is not in the center of the pass band.

The following V+ screen shot shows a SSB radio setup for generic use, using either an off-the–shelf sound card, or SimPhonics SMx system. 

Note the object in the top right corner.  This is the object that sets up the additional modulation parameters for the radio that distinguishes it from a standard generic DIS/HLA radio.

This sample worksheet can be downloaded from SimPhonics web site at: http://www.simphonics.com/library/VPLusSamples/NETWORKED%20AUDIO/

Listening to the Tuning Effect

Using two computers running Windows XP or Windows 2000 and a suitable sound card on both machines (or use SimPhonics SMx system) set up this worksheet mentioned above on both computers.  Use the constant on the R PTT input pin as shown below for Push-To-Talk.  (You can connect this to hardware via one of SimPhonics I/O devices)  Tune one computer’s radio 100 Hz or so from the other, and listen to the effect.  This tuning effect works in real-time as you tune through the incoming signal.