DSP-10 User's Manual

Chapter 3 - Background Information

Material in this chapter adds some details to the operating instructions. This can add to your understanding of the radio;. it may also be important to those that would like to modify the software, or that are "just curious!" The following sections of this Chapter are external to this file, but available by htm link:

Windowing of Spectral Data Understanding more about how to best measure spectra of signals with the DSP-10
Using the ADSP Audio processor The DSP-10 can be operated without the RF hardware

Frequencies in the DSP-10

Several conventions have been used in the DSP-10 to make sure that both ends of a contact can be on the proper frequency! The rules for making this possible and easily done are in the following paragraphs. The description given applies to the display style with a waterfall spectral display (Alt-Y, y). The alternate "slide-rule" display shows a more restricted set of information.

The frequency box on the left side displays the transmit base frequency. For CW, this is the actual transmit frequency. For SSB, this is the frequency that a carrier would be on if it existed. For FSK modes like LHL7, PUA43, EME2 and LTI, it is the base frequency to which the FSK value is added. For these latter modes the FSK value is displayed just above the Transmit Frequency Box as "FSK=xxx."  If the FSK mode involves randomization, the FSK value will change to indicate the current value.

If the path does not involve either EME Doppler correction or randomization, the receive frequency is displayed as follows. For SSB, the displayed frequency in the Transmit Frequency Box is still the carrier frequency. For CW, the Transmit frequency Box is the frequency that will produce a tone to the ear that is equal to "CW Offset" (displayed next to "CW" in the Mode line).  For FSK modes as listed above, the base receive frequency is displayed in the Transmit Frequency Box and the tones are heard at their FSK values, since the receiver is in "USB".

If the receiver frequency has been shifted by RIT, or the path involves EME Doppler corrections, or the mode involves randomization, such as is available in PUA43, EME2 and LTI, the receiver frequency is displayed just below the Transmit Frequency Box, and an "R" (for Random) or "E" (for EME Doppler) is shown to indicate the reason for the receive frequency shift. If only RIT is involved, the receive frequency will be displayed, but no "R" or "E" will be shown since RIT has its own display line.

Note that the Transmit Frequency Box never changes between transmit and receive. If two stations are trying to communicate, they should always have the same frequency displayed in that box.

Also note that the only case where the Transmit Frequency Box changes with mode is CW.  This is a quirk of making shifts between CW and USB painless.  The way it is arranged, a station tuned in on USB will be properly tuned when going to CW, if the CW frequency is offset up in frequency by a proper offset, usually 600 or 800 Hz. This is the scheme used by many, but not all, commercial transceivers.  Two stations using this scheme can receive CW or USB properly regardless of whether they are in either CW or USB modes.

The one situation to avoid is setting the frequency of operation while in the CW mode and then changing to another mode. This will cause a sometimes-puzzling shift by CW Offset. If you do set the frequency in the CW mode, be aware that it is set high by the CW Offset.

DSP Operation - To use the transceiver, it is not necessary to deal directly with the DSP. This is handled by the PC program. However, it is useful to understand the general inner-workings to understand the commands on the PC screen. Most of this must come from comments in the DSP listings for now. But a few ideas are here.

The FFT in the DSP is always 1024 points. The data is arranged to commence a new FFT every 512 data points with 50% overlap on the data sets going to the FFT. This gives some correlation between adjacent FFT outputs which generally only causes extra computation. But, due to windowing of the data, it insures that all input data is used to generate the spectral estimate from the FFT's. Three to twelve successive FFT outputs are power averaged in the DSP before being sent to the PC. This is limited by the serial link. On the PC this is referred to as 1.5, 3 or 6 spectral averages since the data is overlapping and the time period was only long enough for this many independent data sets. Windowing is available in three levels.

The audio FIR filters are 200 taps. In order to not loose accuracy with narrow filters, the output of the filter has 3 steps of gain, 1, 18 and 1/64 as set by the pc when the filter is selected. Any FIR design used here should have a gain of 1, 8 or 64. You choose the highest of the three gains that does not cause coefficients to exceed (0x8000, 0x7FFF).

Several "hooks" allow the changing of the audio and i-f filters, without recompiling the PC computer program.  This always involves changing the coefficients of a FIR filter somewhere in the DSP. There are now tables, and pointers to these tables, that are made available from the DSP program (see that source code).  This will allow changing of the built-in filters as an off-line activity. This can also make "graphic" equalizers available for any filter as well as for the  transmitter audio. Obtaining the address of the various filters occurs at start-up and causes a small delay called "Getting Pointer Table."

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