Making AM Sound Better on an SDR

SAM, SAL/SAU, and SAS — with a KiwiSDR twist

I never really paid much attention to Synchronous AM until I was using my SDR for something other than ham radio. With our EchoTracer3 antenna covering roughly 10 kHz to 1.5 GHz, it’s dangerously easy to start roaming the spectrum. Before long, I was back on medium wave.

On MW I often listen to Radio Caroline on 648 kHz. Another signal we frequently use as a “does my setup behave?” reference is DCF77 on 77.5 kHz — it’s stable, predictable, and makes reception differences easy to spot.

Somewhere in that experimentation, an old truth resurfaced: AM can sound wildly different depending on how you demodulate it. That’s exactly where SAM — and its siblings SAL, SAU, and SAS — become genuinely interesting.

What SAM actually is (and what it isn’t)

In SDR menus, SAM almost always means Synchronous AM (sometimes labeled Sync AM). It does not automatically mean stereo AM.

• Regular AM behaves like an envelope detector. Fine on strong signals, but easily distorted by fading and selective interference.
• Synchronous AM (SAM) uses a PLL-style carrier tracker. It locks onto the carrier, recreates it internally, and demodulates against that clean reference.

The practical result:

• less distortion during fading
• far less “wah-wah” selective fading
• a noticeably more stable, comfortable sound on MW and shortwave

KiwiSDR’s SAM family: SAM, SAL, SAU

On the KiwiSDR, SAM isn’t a single mode. The SAM button cycles through several related modes:

• SAM — classic synchronous AM using both sidebands
• SAL — synchronous AM using lower sideband only
• SAU — synchronous AM using upper sideband only

You can also cycle them quickly using the keyboard:

• Press A to step through SAM → SAL → SAU

Why SAL and SAU matter on medium wave

MW is crowded. Adjacent-channel interference often hits one sideband harder than the other. SAL and SAU let you “dodge” that interference:

• Upper sideband noisy? Try SAL
• Lower sideband messy? Try SAU

You’re still listening to AM, but with the selectivity mindset of SSB — without losing the ease of AM tuning.

Important Kiwi detail: sideband selection overrides the passband

This catches many users at first, but it’s intentional.

If you’re in SAL mode, the entire upper sideband is ignored. That means:

• LSB passband adjustments behave normally
• USB passband adjustments do nothing (because SAU is not used)

The same logic applies in reverse for SAU. Once understood, tuning becomes very predictable.

SAS — Sync AM Stereo (pseudo-stereo done right)

SAS on the Kiwi is best described as Sync AM Stereo, but it is not broadcast AM stereo (C-QUAM).

Instead, SAS works like this:

• SAL → left audio channel
• SAU → right audio channel

On headphones, this creates a surprisingly pleasant spatial effect — especially on music — because the two sidebands are rarely identical once propagation and interference come into play.

It can also help your brain “de-weight” interference. If one sideband is dirty, it often feels less centered and easier to ignore.

SAS is treated as a two-channel mode on the Kiwi, similar to IQ streaming. Expect a short mode-switch delay when entering or leaving SAS.

Try it live:
Listen to Radio Caroline 648 kHz in SAS mode on our KiwiSDR

Watching (and trusting) the SAM PLL

The Kiwi helpfully exposes what the SAM PLL is doing.

Carrier offset display

While in SAM, the PLL carrier offset is shown in the Audio tab.

• Tune exactly on frequency → offset is usually small (often ±10 Hz)
• Click ~1 kHz off in the waterfall → you’ll hear the PLL sweep and re-lock

When SAM loses lock

Lock can drop if:

• the signal is weak
• adjacent-channel interference enters the passband
• the transmitted carrier is badly distorted

A simple recovery workflow:

• retune to center frequency
• switch briefly to AM
• switch back to SAM to encourage a clean re-lock

Tuning the PLL itself

In the Audio tab, the PLL time constant can be adjusted:

• DX — slower, more stable lock for weak or fading signals
• Reset — forces a fresh lock attempt

On difficult nighttime paths — such as 648 kHz under heavy fading — the DX setting can be the difference between “almost listenable” and “actually enjoyable.”

Real AM stereo: C-QUAM (QAM mode)

SAS is pseudo-stereo. The Kiwi can also decode true AM stereo if the station transmits C-QUAM.

On the Kiwi:

• Cycle the SAM button until QAM appears
  (or use uppercase A)

If the station genuinely transmits C-QUAM, switching between AM and QAM will reveal real stereo separation.

C-QUAM stations still exist, but they are far rarer than in the past. Daytime reception is often more reliable.

A practical listening ladder (Radio Caroline 648 example)

1. AM — baseline reference
2. SAM — reduced warble and fading distortion
3. SAL / SAU — pick the cleaner sideband
4. SAS — headphones, spatial sideband awareness
5. QAM — only if true AM stereo is present

Why this matters

The Kiwi’s SAM family is a perfect reminder that demodulation is half the radio.

With a wide-coverage antenna and a few known signals, you can clearly hear:

• how fading breaks envelope AM
• how synchronous detection stabilizes it
• how sideband selection beats interference
• how SAS reveals sideband differences
• how C-QUAM is a separate (and rarer) world

Explore and listen yourself:
SDR Receivers and Accessories for Serious Listening

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