Sample Rates

The Apollo hardware supports six sample rates across three speed tiers. This page documents the supported rates, how to change them, clock source configuration, and important considerations for rate switching.

Supported Sample Rates

The default sample rate at driver load is 48,000 Hz.

How to Change Sample Rate

Via ALSA Control

The driver exposes a Sample Rate ALSA control:

# View current rate
amixer -c Apollo cget name='Sample Rate'

# Set to 96 kHz (index 3, zero-based in ALSA enum)
amixer -c Apollo cset name='Sample Rate' 3

The ALSA enum values map as follows:

Via PipeWire

PipeWire can request a specific rate when opening a stream. The driver handles rate changes in its PCM prepare callback. To set PipeWire's default rate:

mkdir -p ~/.config/pipewire/pipewire.conf.d
cat > ~/.config/pipewire/pipewire.conf.d/99-rate.conf <<EOF
context.properties = {
    default.clock.rate = 48000
    default.clock.allowed-rates = [ 44100 48000 88200 96000 176400 192000 ]
}
EOF
systemctl --user restart pipewire

With default.clock.allowed-rates set, PipeWire can automatically switch the hardware rate to match the content being played.

Via the Mixer Daemon

The mixer daemon accepts sample rate changes through the TCP:4710 and WS:4721 control protocols, which are used by the console UI.

Clock Source Configuration

The Apollo can synchronize to an internal clock or an external clock reference. The driver exposes two ALSA controls for clock configuration.

Clock Source

Selects where the word clock comes from:

amixer -c Apollo cget name='Clock Source'
amixer -c Apollo cset name='Clock Source' 0  # Internal

Clock Mode

Selects between internal and external clocking modes:

amixer -c Apollo cget name='Clock Mode'
amixer -c Apollo cset name='Clock Mode' 0  # Internal

When using an external clock source, the Apollo locks to the incoming clock signal. If no valid external clock is detected, audio may not function correctly.

How Rate Switching Works

Understanding the rate switching mechanism helps diagnose issues.

Clock Configuration Register

The driver writes a combined clock configuration value to the hardware:

clock_config = clock_source | (rate_index << 8)

For example, 48 kHz with internal clock: 0x00 | (2 << 8) = 0x0200.

Note: The special value 0x0C is used during initialization to activate DSP settings processing and is not the steady-state internal clock value.

This value is written to the notification clock config register, followed by a clock doorbell write to signal the FPGA. The driver then waits up to 2 seconds for the DSP to acknowledge the rate change via the notification status register.

Rate Change During Playback

When PipeWire or an ALSA application requests a different rate during playback, the driver performs a disconnect/reconnect cycle:

1. Stop the period timer
2. Soft disconnect the audio transport (stops DMA, preserves mixer DSP state)
3. Update the internal rate tracking
4. Reconnect the audio transport at the new rate
5. Restart transport and clock

This process takes a brief moment and will cause an audible gap. Applications should stop playback before switching rates when possible.

Rate Change While Idle

When no streams are open, changing the rate via the ALSA control writes the new clock configuration immediately without a disconnect cycle.

Channel Count at Higher Rates

The Apollo's PCIe DMA channel count is fixed regardless of sample rate. The driver always allocates the full channel count for your model (e.g., 24 playback / 22 capture on the Apollo x4).

However, at higher sample rates, the Apollo's internal DSP processing capacity is reduced. At 2x rates (88.2/96 kHz), some internal mixer channels may be unavailable. At 4x rates (176.4/192 kHz), the available DSP channels are further reduced.

The physical I/O availability at each tier depends on the specific Apollo model:

ADAT operates in S/MUX mode at double-speed rates, halving the available channel count. At quad-speed rates, ADAT is not available.

Best Practices

• Set the rate before opening streams. Changing rates while audio is active causes a brief dropout.
• Match your project rate. If your DAW session is at 96 kHz, set the Apollo to 96 kHz before starting the session.
• Use 48 kHz as default. This is the most compatible rate and the driver's default. It provides the full channel count and lowest CPU overhead.
• External clock requires a valid signal. When switching to an external clock source, ensure the external device is connected and sending a valid clock at the expected rate before switching.
• PipeWire rate switching can be configured to automatically match content rates using default.clock.allowed-rates, but this triggers a full disconnect/reconnect cycle on each rate change.