Configuring the PowerSDR for optimum DRM and Digital Voice
Configuring the PowerSDR for optimum DRM and Digital Voice
| System Dependencies
| Minimum PowerSDR Version:
| Applicable Hardware:
|| All transceivers
Content provided by: Mel, KØPFX
Digital Radio Mondiale or DRM (www.drm.org) on the shortwave bands is easily decoded using the Dream software receiver (http://sourceforge.net/projects/drm/) and PowerSDR’s DRM mode. PowerSDR and Dream may also be used as an experimental transmitter for digital audio, text messages and images. The PowerSDR’s DRM with Dream has a decoder for analog modes including AM, LSB, USB, CW, and FM. With this dynamic duo, today’s shortwave listener has it all in one easy to use and versatile package. And it is just plain fun to operate! As shown above, select GEN, DRM, VAC, 48K Sample rate and check Stereo. Execute Dream and perform the following set up.
Virtual Audio Cable http://software.muzychenko.net/eng/vac.html is required for Sound In. VAC replaces the requirement for a second card. Select M-Audio Delta 44 Multi (or M-Audio Delta 44 1/2). To avoid hearing the high frequency IF tone, click on MUTe in the PowerSDR console. For VAC set up see:
http://test.flexradio.com/article.aspx?id=10218 for VAC 4.03
DRM from the top end of the 75m band 3995kHz
Here Dream is shown receiving audio from the VAC Virtual cable 2. Adjust the VAC RX gain in the PowerSDR Setup for a level of approximately -25dB. My experience has found DRM performs with minimum interruptions (dropouts) when the received signal strength is over S-9 (SNR of 20+ dB). DRM’s Tx mode and data rate of the signal (set by the transmitting station) directly affect the signal strength requirement for the Dream receiver. One of the better web sites for DRM (SWL and technical description) is found at www.drmradio.co.uk.
Experimenting with DRM at 10 kHz spectrum. Transmitting 22.14 kbps
Here is the Analog mode of Dream (hot key: Ctrl A). Tune from the SDR or just left click on the signal here (mouse will turn to a “+” when in the spectrum). The DSP filter works quite well, especially for SSB and CW.
For experimental transmitting into a dummy load, the SDR is set up for SPLIT SSB operation and the VFO B’s transmit frequency is lowered to match the DC Offset of the DRM receiver (12KHz). The SDR will then be transmitting and receiving on the same frequency. DRM uses OFDM and its crest factor is quite high (around 9 dB) so keep the average power less than 15 watts out. Wide BW modes are within the FCC rules on certain segments of 6 meters and above in US.
Adjust the band pass to center the 10kHz OFDM carrier. The audio data stream from the second sound card output to the M-Audio card will trip the PowerSDR VOX into transmit when clicking on Dream’s “Start” button.
This is the DREAM transmitter sending data (jpg pix) and a text message. Check Audio Enable for voice and use a microphone connected to the second sound card. I use a small Behringer UB502 mixer and a Heil PR40 microphone however, a low cost PC electret will work Ok. The Dream transmitter is executed from the batch file “transmitter.bat”. This bat file uses the Dream executable –t option (dream –t). See the docs for the long list of options and arguments for both the receiver and transmitter (help.bat in Dream directory).
Second sound card selected for Microphone audio input to be encoded. Use this card’s mixer control (Recording MIC in) to adjust the Dream Audio level as shown in the above indicator while speaking into the microphone.
Second sound card selected for encoded audio out. Open up the second sound card’s mixer and use the Mixer’s Playback speaker and wave out to adjust the level. The speaker output of this card is connected to the M-Audio Delta 44 3/4 input. Use this adjustable level and the PowerSDR Console’s MIC gain to set the RF output.
“SPARK” found at http://www.drm-sender.de/ is a software transmitter that may be used to experiment with the various functions of DRM. It requires a second sound card for audio in (and has other data input handling capabilities). VAC may be used for data out to SDR. Check with the author for the latest releases and functionality.
PowerSDR and Digital Voice/Data using WinDRM
PowerSDR and WinDRM (http://n1su.com/windrm/docs/1.1/) provide good voice quality audio without the QRN and QSB normally experienced over HF. This updated configuration card requires only one sound card. In this setup, the popular M-Audio Delta 44 and VAC are used. For PTT control, use N8VB’s Virtual Serial Port driver software found here http://www.philcovington.com/SDR.html. Plug the station microphone into the Delta 44 3/4 input. A low cost Behringer 502 preamp/mixer or equivalent may be needed because the SDR’s Equalizer/preamp can not be used in this application. Plug a set of amplified PC speakers (or headphones) for the decoded voice audio into the Delta 44 1/2 output. VAC takes care of the receive input audio and the transmit audio. Click on MUT to eliminate hearing the data stream while decoding voice.
Set the Transmit Filter bandwidth as shown. WinDRM requires 2.5KHz of spectrum with a 350Hz DC offset. Make final adjustments and center within the bandpass using the mouse while transmitting. Do not use compression, compander or the EQ.
Transmitting Digital Voice SDR PTT Control
The receive SNR info during transmit indicates BPS of the Main Service Channel (voice data). This display is showing mic input level while speaking. For PTT control, enable SDR’s PTT. In this configuration, WinDRM is using Com 4 and SDR Com 5. Do not check any other options in the WinDRM PTT port menu.
Receiving Digital Voice
Select sound card and VAC as shown. Best audio is obtained using the MELP codec. An SNR of 10 dB or higher will provide good communications voice audio with no noise. In the data mode, data rates of 6+ kbs maybe realized for file transfers. 14.236 is a calling frequency for DV. 14.233 is the hamDRM (digital-SSTV) picture transfer frequency.
PowerSDR and Digital Voice with AOR modems
Setup for SSB operation and a bandwidth for the flat-top OFDM carrier signal as shown. Signal looks/sounds very similar to WinDRM but has a distinctive pre-amble/sync tone at the start of each transmission. On transmit, set for approximately 30 watts output, carrier peaks will be around 100w. VAC is not required and must be off. MUTe the incoming audio to avoid hearing the “digital noise”. Decoded voice audio will be heard in the AOR speaker. To check the outgoing digital audio, use the PowerSDR’s MONitor.
Connect Data OUT from the AOR “Radio” connector pins 1and 2 to the audio input of the transceiver. Connect PTT control pins 3 and 4 to the the transceivers microphone PTT control. For audio IN, connect AOR “SP” IN to the output of the PC’s speakers. For this audio connection, you may use the headphone’s jack found on most powered PC speakers. The audio may then be set for the proper level into the AOR using its volume control or the PowerSDR AF. A separate speaker is recommended for the AOR decoded voice. When the AOR is not in use, simply disconnect the plug at the PC speaker’s headphone jack. The AOR has a “straight thru” analog/ssb mode for the PowerSDR’s compander, compressor, and EQ. These should not be used in the DV mode. If any common mode noise (ground loop) is found to affect audio, try a ground loop isolator (1:1 transformer) in the In/Out audio lines (www.solorb.com/elect/musiccirc/stereoiso/index.html). A ground wire between the AOR and the rig is recommended.
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Last Modified:Friday, October 10, 2008
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