Making AM Sound Better on an SDR

I did not pay much attention to synchronous AM while staying inside the amateur bands. Then I started roaming medium wave and long wave with an SDR. Radio Caroline on 648 kHz and DCF77 on 77.5 kHz became familiar reference signals, and the demodulator suddenly mattered as much as the antenna.

AM can sound completely different without changing the RF signal. Envelope AM, synchronous double-sideband AM, synchronous single-sideband reception and true AM stereo ask the same samples different questions.

Why Ordinary AM Warbles During Selective Fading

A conventional AM detector follows the received envelope. When multipath propagation changes the carrier and sidebands by different amounts or phases, the envelope can become badly distorted even though the transmission is still present.

A synchronous detector estimates the carrier’s frequency and phase, recreates a local reference and uses it for demodulation. That can remove much of the carrier-related “wah-wah” distortion. It cannot restore a sideband that propagation or interference has erased, and it can lose lock when the carrier is too weak or contaminated.

SAM Uses Both Sidebands

On KiwiSDR, SAM is the traditional double-sideband synchronous detector. It is the first mode to try when an AM signal is centred correctly but envelope detection sounds unstable.

Watch the carrier-offset indication in the Audio panel. A small stable offset near the tuned centre suggests that the PLL is tracking. If the sound sweeps, breaks up or the offset wanders wildly, centre the carrier and use the PLL reset. The Kiwi’s DX time-constant setting can make weak-signal lock steadier, with the trade-off that it responds more slowly.

SAL and SAU Choose the Cleaner Sideband

SAL demodulates the lower sideband synchronously; SAU uses the upper sideband. This is useful on crowded medium wave, where an adjacent station or selective fade may damage one sideband more than the other.

• If interference sits above the wanted carrier, try SAL.
• If interference sits below it, try SAU.
• If both sides are clean, compare SAM for the fuller double-sideband result.

KiwiSDR deliberately lets the selected sideband override the opposite half of the visible passband. In SAL, changing the upper-sideband edge has no effect because that sideband is not being demodulated. SAU behaves the same way in reverse.

SAS Is Sideband Pseudo-Stereo

SAS routes the synchronously detected lower sideband to one audio channel and the upper sideband to the other. It does not recreate studio stereo. It makes propagation and interference differences between the sidebands audible as spatial differences.

On headphones, I find that surprisingly pleasant with music. A dirty sideband often moves away from the centre, which can make the wanted programme easier to follow. Because SAS is a two-channel mode, KiwiSDR has a short switching delay when entering or leaving it.

QAM Is for Real C-QUAM Stereo

KiwiSDR’s QAM mode decodes C-QUAM AM stereo when the station actually transmits it. That is a different system from SAS. With a mono AM transmission, QAM cannot invent the original left and right programme channels.

The official KiwiSDR instructions use uppercase A to reach QAM and lowercase a to return through the ordinary AM modes. Confirm stereo by comparing AM and QAM on a known C-QUAM station rather than assuming that any spacious sound is encoded stereo.

A Radio Caroline Listening Ladder

Radio Caroline’s official site confirms its 648 kHz AM service. The public RF.Guru KiwiSDR link was also reachable during this review, so the original experiment remains available:

Listen to Radio Caroline 648 kHz with SAS preselected on the RF.Guru KiwiSDR.

• AM: establish the ordinary envelope-detector baseline.
• SAM: compare carrier stability and fading distortion.
• SAL: listen only to the lower sideband.
• SAU: listen only to the upper sideband.
• SAS: place those sidebands in separate ears.
• QAM: use only when checking a genuine C-QUAM transmission.

Keep RF bandwidth, audio bandwidth, AGC and volume comparable while switching. Otherwise a louder or wider mode can seem “better” for reasons unrelated to demodulation.

When SAM Will Not Stay Locked

• Centre the carrier accurately.
• Narrow the passband enough to exclude the interfering neighbour without cutting wanted audio unnecessarily.
• Try SAL or SAU when one sideband is damaged.
• Select the DX PLL time constant for a weak or fading signal.
• Use Reset, or switch briefly to AM and back to SAM, to force a clean acquisition.
• Return to ordinary AM when the PLL hunts more than it helps.

Synchronous detection is not automatically better. A strong, clean AM signal can sound excellent in ordinary AM. Co-channel interference, severe sideband loss, receiver overload and local noise remain RF problems that a PLL cannot solve.

Primary and Authoritative Sources

• KiwiSDR Operating Information—official SAM, SAL, SAU, SAS, QAM, passband, carrier-offset and PLL controls.
• KiwiSDR source repository—current project, receiver architecture and software history.
• Radio Caroline’s official 648 AM page—the station and frequency used in Joeri’s listening example.

For other receivers and accessories, see SDR Receivers and Accessories for Serious Listening.

Joeri’s Bottom Line

The antenna gets the signal into the receiver. The demodulator decides what you hear from it.

Use SAM to steady the carrier, SAL or SAU to escape a damaged sideband, SAS to hear the sidebands separately and QAM only for genuine C-QUAM. Once you listen that way, medium wave becomes a laboratory for propagation rather than background audio.

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