Digital signal processing
| Digital signal processing | |
|---|---|
| Abbreviation | DSP |
| What it is | Processing radio or audio signals as streams of numbers |
| Key parts | Analog-to-digital converter, processor (DSP chip, FPGA or computer), digital-to-analog converter |
| Sampling rule | Sample at least twice the highest frequency present |
| In ham radio | |
| Receive uses | Filtering, noise reduction, notch filters, demodulation, AGC |
| Transmit uses | Modulation, speech processing, equalization |
| Digital modes | Generated and decoded in software, as with FT8, PSK31 and FreeDV |
| Early ham products | Add-on audio filters such as the JPS NIR-10 and Timewave DSP-9, 1990s |
Digital signal processing, or DSP, is the handling of signals as numbers. A converter measures a signal's voltage thousands or millions of times a second, a processor does arithmetic on the resulting stream of numbers, and, if a listener needs to hear the result, another converter turns the numbers back into audio. Almost everything a receiver or transmitter does, including filtering, mixing, detecting and modulating, has a digital equivalent. DSP first reached most ham shacks as add-on audio filters in the 1990s; today it is at the heart of most new transceivers and of every sound-card digital mode.
Sampling
The first step is the analog-to-digital converter (ADC), which samples the signal at regular intervals. The Nyquist criterion says the sampling rate must be at least twice the highest frequency in the signal.[1] If it is not, higher frequencies fold back and appear as false signals at lower frequencies, an effect called aliasing, and once that has happened no later processing can remove them.[1] So every ADC is preceded by an anti-aliasing filter that removes frequencies above the range of interest.[1]
For audio work, a sound card sampling at 48,000 times a second covers everything an SSB receiver passes. A direct-sampling software-defined radio samples the antenna signal itself, at a rate high enough to cover the HF bands in one go. The number of bits in each sample sets the converter's dynamic range, the span between the strongest signal it can take without overloading and the weakest it can resolve, one part of receiver performance.
Digital filters
The best-known use of DSP in a ham radio is the filter. Two main families are used.
- FIR (finite impulse response) filters combine the most recent samples, each multiplied by a fixed coefficient. They have no feedback, so they are always stable, and they can be designed with linear phase, meaning all frequencies are delayed equally, which keeps CW and data signals clean. Sharp FIR filters need many coefficients.[2]
- IIR (infinite impulse response) filters also feed part of their output back in. They reach a given sharpness with far fewer calculations, but the phase response is not linear and a poorly designed one can become unstable.[2]
DSP filters can have very steep skirts, often called "brick-wall" filters, and because they are only numbers in a program, a radio can offer dozens of bandwidths and let the operator shift or reshape the passband with a knob. The cost is a short delay, since a sharp filter has to collect samples before it produces output.
Noise reduction and notch filters
DSP made adaptive processing practical. An automatic notch filter finds a steady tone, such as a carrier on top of an SSB conversation, and removes it. Noise reduction uses the fact that speech and CW tones are partly predictable from moment to moment while random noise is not, and suppresses the unpredictable part. Neither can recover a signal that is truly buried, and too much of either distorts the wanted one.
DSP in the radio
DSP has moved steadily closer to the antenna.
- Audio DSP processes the receiver's speaker output, as the early add-on filters did and as many radios still do for noise reduction.
- IF DSP digitizes the receiver's intermediate frequency, so the main selectivity filters are digital.
- Direct sampling digitizes the RF signal itself. Icom's IC-7300 brought this design to an entry-level HF radio; the antenna signal reaches the converter without first being mixed to a lower frequency, and an FPGA, a programmable chip that does fast arithmetic in parallel, does the processing.[3] A direct-sampling radio can show a whole band on a panadapter at once.
In transmitters, DSP generates the modulation, shapes CW keying, and handles speech processing and equalization. Some recent radios use predistortion, deliberately distorting the drive in the opposite direction to cancel the distortion of the power amplifier.[4] Loops such as AGC are often digital as well.
Sound-card digital modes are DSP in its purest form. Programs such as WSJT-X for FT8, fldigi for PSK31 and RTTY, and FreeDV for digital voice do all their modulation and decoding in software on a general-purpose computer.
History
QST described digital processing for experimenters in a November 1984 article, "Digital Signal Processing for the Experimenter," and a 1991 QST introduction to DSP noted that the technique had become part of amateur radio.[5] By the mid-1990s, external DSP audio processors were in amateur use. The JPS NIR-10 was in use by early 1994, when owners were already debating how well its noise reduction worked,[6] and Timewave Technology of St. Paul, Minnesota, published the manual for its DSP-9 audio noise filter the same year.[7] The DSP-9 offered adaptive random-noise reduction, an automatic notch for heterodynes and selectable voice and CW filters.[7] By the mid-1990s DSP had also moved inside transceivers. In 1998 QEX ran a four-part tutorial, "Signals, Samples and Stuff," whose second part described an IF-DSP transceiver.[5]
See also
References
- ↑ 1.0 1.1 1.2 What the Nyquist Criterion Means to Your Sampled Data System Design, Analog Devices.
- ↑ 2.0 2.1 Mixed-Signal and DSP Design Techniques, Section 6: Digital Filters, Analog Devices.
- ↑ IC-7300, Icom America.
- ↑ ARRL's Clean Signal Initiative Adopts Metrics for Transmitter Performance, QST, May 2024.
- ↑ 5.0 5.1 DSP - Digital Signal Processing, ARRL.
- ↑ Info-Hams Digest V94 #236, March 1994.
- ↑ 7.0 7.1 DSP-9 Audio Noise Reduction Filter manual, Timewave Technology, 1994.