google/ExoPlayer · error · IllegalArgumentException

Coefficient at index {i} is negative.

Error message

Coefficient at index {i} is negative.

What it means

ChannelMixingMatrix validates user-supplied coefficient arrays with checkCoefficientsValid and throws IllegalArgumentException for any negative value — every coefficient must be >= 0f. The restriction exists because the mixing implementation assumes non-negative gains (it cannot express phase inversion); negative gains such as the -0.707 used in some surround-downmix recipes are rejected.

Source

Thrown at library/common/src/main/java/com/google/android/exoplayer2/audio/ChannelMixingMatrix.java:192

        "Default channel mixing coefficients for "
            + inputChannelCount
            + "->"
            + outputChannelCount
            + " are not yet implemented.");
  }

  private static float[] initializeIdentityMatrix(int channelCount) {
    float[] coefficients = new float[channelCount * channelCount];
    for (int c = 0; c < channelCount; c++) {
      coefficients[channelCount * c + c] = 1f;
    }
    return coefficients;
  }

  private static float[] checkCoefficientsValid(float[] coefficients) {
    for (int i = 0; i < coefficients.length; i++) {
      if (coefficients[i] < 0f) {
        throw new IllegalArgumentException("Coefficient at index " + i + " is negative.");
      }
    }
    return coefficients;
  }
}

View on GitHub (pinned to dd430f7053)

Solutions

1. Clamp or zero out negative coefficients: use Math.abs(coef) only if phase truly does not matter for your use, otherwise set surround contributions to 0f or +gain.
2. Do phase inversion in a different processor that supports it (e.g. a custom AudioProcessor), keeping the ChannelMixingMatrix coefficients non-negative.
3. Validate the array yourself before construction to fail with a clearer message.

Example fix

// before
float[] bs775 = {1,0, 0,1, .707f,.707f, 0,0, -.707f,0, 0,-.707f};
ChannelMixingMatrix.create(6, 2, bs775); // throws IllegalArgumentException at index 8

// after
float[] nonNeg = new float[bs775.length];
for (int i = 0; i < bs775.length; i++) nonNeg[i] = Math.max(0f, bs775[i]);
ChannelMixingMatrix.create(6, 2, nonNeg);

private static float[] requireNonNegative(float[] coefficients) {
  for (int i = 0; i < coefficients.length; i++) {
    if (coefficients[i] < 0f) {
      throw new IllegalArgumentException("Coefficient[" + i + "]=" + coefficients[i] + " must be >= 0");
    }
  }
  return coefficients;
}
float[] safe = requireNonNegative(coeffs);
ChannelMixingMatrix.create(in, out, safe);

When it happens

Trigger: Passing a float[] containing any value < 0f to the ChannelMixingMatrix constructor (create(input, output, coefficients)) — most commonly a copy of ITU-R BS.775 downmix coefficients that use -0.7071 for surround channels.

Common situations: Porting a downmix matrix from another DSP library that permits negative (phase-inverting) gains; typo'd or wrap-around index math producing spurious negative floats.

Related errors

• Unhandled input format: {audioFormat}
• No mixing matrix for input channel count {audioFormat}
• Default channel mixing coefficients for {inputChannelCount}-
• Unhandled input format: ${audioFormat}
• Unhandled input format: {audioFormat}

AI-assisted analysis of google/ExoPlayer@dd430f7053 (2026-08-14). Data as JSON: /api/errors/8be513e17a97dfb4. Report an issue: GitHub.