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Audio: volume, stereo and the Amiga's filters

AExp plays the Amiga's sound on HDMI and on the MEGA65's 3.5 mm audio jack at the same time, and every setting described here affects both outputs. The one exception is the DVI (no sound) option, which switches the HDMI audio off and leaves the jack.

All settings are in the "Audio" section of the options menu (press Help). Out of the box, AExp sounds like a real Amiga 500, including the machine's two audio filters. One of them is coupled to the power LED, for reasons explained below.

Volume

The volume control works in 5% steps, and the percentages describe what you hear: 50% sounds half as loud as 100%, 25% a quarter as loud. At the default of 100% the sound passes through unchanged, and 0% is a true mute. The control works like the volume knob on your monitor: it sits outside the simulated Amiga, so games and demos cannot tell it is there.

Stereo Mix

The Amiga has four sound channels, wired in a way that was common in the 1980s: channels 1 and 4 play only on the left, channels 2 and 3 only on the right. On desk speakers this sounds fine, because the room blends the two sides. Headphones have no room, and an instrument that plays only in your left ear while the drums play only in your right gets tiring quickly. Most Amiga music was not mixed for headphones.

The Stereo Mix setting blends some of each side into the other, the same way the MiSTer Amiga core does:

  • Full Stereo: the authentic hard-panned output (the default).
  • Wide Stereo: a gentle blend (87.5% own side, 12.5% opposite side).
  • Narrow Stereo: a stronger blend (75% / 25%).
  • Mono: both channels merged, for single-speaker setups.

On headphones, try Wide or Narrow Stereo: the music keeps its direction but is easier on the ears.

A500 Filter

Paula, the Amiga's sound chip, plays digital samples: a rapid staircase of discrete values. A real A500 never sends that staircase to the line output directly. An always-on analog low-pass filter, rolling off gently above roughly 4.4 kHz, first rounds off the hardest edges. That filter is a large part of the warm, slightly soft sound people remember, and the musicians of the time composed with it in place.

The A500 Filter switch recreates that filter and is on by default. Switching it off removes the fixed filter from the path, which is essentially what Commodore did in the A1200: it shipped without this filter and is known for its brighter, crisper sound. On gives you the classic A500, off an A1200-like sound.

LED Filter

Besides the fixed filter, the Amiga contains a second, much stronger low-pass filter (it cuts in around 3 kHz) that software can switch on and off at any time. A switchable filter needs a control line, and the pins of the Amiga's I/O chips were scarce. So Commodore connected the filter to a signal that already existed: the one that sets the brightness of the power LED.

On every real A500, therefore, power LED bright means filter on, power LED dimmed means filter off. When a game or demo switches the filter off for brighter music, the power LED dims at the same moment. Amiga musicians used this on purpose: ProTracker exposes it as its FILTER setting, songs can toggle it mid-tune with a command, and many games switch it off when their title music starts. After a reset the filter is on (LED bright) until software changes it.

The LED Filter switch in the menu decides whether AExp honors this mechanism. On (the default), the simulated Amiga behaves like real hardware: the running software decides, live, whether the filter is in the audio path. Off, the filter never engages, whatever the software does. The MEGA65's own power LED does not mirror the simulated one, so you hear the filter switch but the light stays as it is.

Which settings should I use?

  • Authentic A500: the defaults, A500 Filter on, LED Filter on, Full Stereo. This is how an A500 sounded in 1989.
  • Bright and modern: A500 Filter off, LED Filter off. This is the raw, unfiltered output of the sound chip, crisper and more "digital" than any real A500 sounded through its own output stage.
  • Headphones: whatever else you choose, set Stereo Mix to Wide or Narrow Stereo.

Like all menu settings, the audio configuration is saved on the SD card automatically, so your choice survives switching the core off and on again.