Narrow band modes such as PSK31 are low symbol rate, single carrier differential Binary PSK (called 2-PSK or BPSK) or Quadrature PSK (4-PSK or QPSK). With digital phase modulation the phase changes abruptly, and without additional measures wide sidebands would be created. To prevent this, all these modes also include 100% raised-cosine amplitude modulation (ASK) at the symbol rate, which reduces the power to zero at the phase change.
Because of this amplitude modulation, the signal bandwidth is relatively narrow. Synchronization at the receiver is straightforward because it can be recovered from the amplitude information. Differential PSK is used to provide continuous phase changes when idle (to maintain sync), and by allowing the receiver to measure phase difference from symbol to symbol, to reduce the effects of ionospheric Doppler phase changes which modulate the signal. The slower modes are more affected by Doppler, and the QPSK modes are particularly affected.
With no interleaver and limited coding length, the QPSK mode Forward Error Correction coding gain is limited, and under burst noise conditions (HF) the performance is usually worse than the BPSK option at the same baud rate. In general the narrow-band BPSK modes work well on a quiet single-hop path, but give poor performance in most other conditions.
With these modes, a very linear transmitter is required. Over-driven operation results in excessive bandwidth, poorer reception and difficult tuning. However, the sensitivity is such that very little power is usually required.
These are unconnected, manually controlled message asynchronous simplex chat modes, used without Forward Error Correction. The FEC option is rarely used. The default calling mode is BPSK31.
A binary varicode with ASCII-256 user interface is used. Lower case characters are sent faster. Modulation is bit-wise symbol synchronous, differential.
The QPSK modes use binary convolution to generate two dibits per varicode bit at the same symbol rate. Rate R=1/2, Constraint length K=5. No interleaver is used. Two-bit quadrature modulation is based on a differential code table.
| Mode | Symbol Rate | Typing Speed1 | Duty Cycle2 | Bandwidth3 | ITU Designation4 |
| BPSK315 | 31.25 baud | ~ 3.5 cps (35 wpm) | ~ 80% | 62.5 Hz | 63H0G1B |
| BPSK63 | 62.5 baud | ~ 7.0 cps (70 wpm) | ~ 80% | 125 Hz | 125HG1B |
| BPSK125 | 125 baud | ~ 14.0 cps (140 wpm) | ~ 80% | 250 Hz | 250HG1B |
| BPSK250 | 250 baud | ~ 28.0 cps (280 wpm) | ~ 80% | 500 Hz | 500HG1B |
| QPSK31 | 31.25 baud | ~ 3.5 cps (35 wpm) | ~ 80% | 62.5 Hz | 63H0G1B |
| QPSK63 | 62.5 baud | ~ 7.0 cps (70 wpm) | ~ 80% | 125 Hz | 125HG1B |
| QPSK125 | 125 baud | ~ 14.0 cps (140 wpm) | ~ 80% | 250 Hz | 250HG1B |
| QPSK250 | 250 baud | ~ 28.0 cps (280 wpm) | ~ 80% | 500 Hz | 500HG1B |
1. WPM is based on an average 5 characters per word, plus word space. Values are approximate because a variable length code is used.
2. Transmitter average power output relative to a constant carrier of the same PEP value.
3. This is the "Necessary Bandwidth" as defined by the ITU.
4. A summary of the ITU Designation system can be found at