What causes "pcm: unsupported audio format: ${audio}" in AlexxIT/go2rtc?

Check the audio string against the formats core.ParseCodecString accepts (e.g. "pcml/16000", "pcma/8000")

pcm: unsupported audio format

Error message

pcm: unsupported audio format: ${audio}

What it means

NewBackchannel builds a PCM backchannel and needs a codec. It takes the audio format string, tries core.ParseCodecString on it, and throws this error when the string cannot be parsed into a known codec (and the caller did not leave it empty to get the default 16kHz PCML codec).

Solutions

Check the audio string against the formats core.ParseCodecString accepts (e.g. "pcml/16000", "pcma/8000")
Pass an empty string to get the default codec (PCML, 16000 Hz)
Log the exact value of the audio parameter coming from your caller/config and fix the typo

Example fix

// before
bc, err := NewBackchannel("pcm_l16")
// after
bc, err := NewBackchannel("pcml/16000") // or NewBackchannel("") for the default

Defensive patterns

Strategy: validation

Validate before calling

if audio != "" && core.ParseCodecString(audio) == nil {
    return fmt.Errorf("unsupported audio format %q; use pcml/pcma or empty for default", audio)
}
bc, err := NewBackchannel(audio)

Try / catch

if err != nil {
    if strings.Contains(err.Error(), "unsupported audio format") {
        bc, err = NewBackchannel("") // fall back to default codec
    }
}

Prevention

Whitelist allowed codec strings in your config layer before passing them in
Use the empty-string default when unsure of the exact format
Unit-test any codec string that comes from user input

When it happens

Trigger: Calling NewBackchannel (via execHandle) with a non-empty `audio` string that core.ParseCodecString cannot parse, e.g. "pcm-ulaw" or a misspelled codec name like "pcml " or "opus/48000/2" if unsupported.

Common situations: Typo in codec name in config or URL query parameter; passing an SDP-format codec string when a simple name is expected; assuming formats like opus are supported when only PCM variants are.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of AlexxIT/go2rtc@c245815e75 (2026-09-07). Data as JSON: /api/errors/7fb034f2a87f208e. Report an issue: GitHub.

Appendix: source

Thrown at pkg/pcm/backchannel.go:23

	"github.com/AlexxIT/go2rtc/pkg/core"
	"github.com/AlexxIT/go2rtc/pkg/shell"
	"github.com/pion/rtp"
)

type Backchannel struct {
	core.Connection
	cmd *shell.Command
}

func NewBackchannel(cmd *shell.Command, audio string) (core.Producer, error) {
	var codec *core.Codec

	if audio == "" {
		// default codec
		codec = &core.Codec{Name: core.CodecPCML, ClockRate: 16000}
	} else if codec = core.ParseCodecString(audio); codec == nil {
		return nil, errors.New("pcm: unsupported audio format: " + audio)
	}

	medias := []*core.Media{
		{
			Kind:      core.KindAudio,
			Direction: core.DirectionSendonly,
			Codecs:    []*core.Codec{codec},
		},
	}

	return &Backchannel{
		Connection: core.Connection{
			ID:         core.NewID(),
			FormatName: "pcm",
			Protocol:   "pipe",
			Medias:     medias,
			Transport:  cmd,
		},

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