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BinauralQualityProbe.kt

src/jvmMain/kotlin/afield/examples/steamaudio/BinauralQualityProbe.kt · 368 lines

package afield.examples.steamaudio

import afield.audio.spatial.steamaudio.AmbisonicsDecodeEffect
import afield.audio.spatial.steamaudio.AmbisonicsDecodeEffectParams
import afield.audio.spatial.steamaudio.AmbisonicsEncodeEffect
import afield.audio.spatial.steamaudio.AmbisonicsEncodeEffectParams
import afield.audio.spatial.steamaudio.AudioBuffer
import afield.audio.spatial.steamaudio.AudioSettings
import afield.audio.spatial.steamaudio.BinauralEffect
import afield.audio.spatial.steamaudio.BinauralEffectParams
import afield.audio.spatial.steamaudio.Hrtf
import afield.audio.spatial.steamaudio.HrtfInterpolation
import afield.audio.spatial.steamaudio.SteamAudio
import afield.audio.spatial.steamaudio.SteamAudioContext
import java.lang.foreign.Arena
import kotlin.math.PI
import kotlin.math.abs
import kotlin.math.cos
import kotlin.math.log10
import kotlin.math.sin
import kotlin.math.sqrt

Measures what actually improves binaural presentation, so the quality settings can be chosen from evidence rather than from the assumption that bigger numbers are better.

Three questions, each answered by rendering impulses and measuring the result:

  1. Does HRTF interpolation matter? NEAREST snaps to the closest measured direction, so a smoothly moving source should show level steps. BILINEAR blends neighbours. Measured as the largest ear-balance jump between adjacent 2-degree steps: a big jump is an audible zipper as a source moves.
  2. What Ambisonic order does the reflected path need? Reflections arrive as Ambisonics and are decoded binaurally, and that round trip blurs direction. Measured against direct binaural rendering -- the accuracy ceiling -- at orders 1, 2 and 3.
  3. How much does spatial blend cost? Pulling a source toward the centre of the head is sometimes wanted, but it trades away the localisation cues.
private const val SAMPLING_RATE = 48_000
private const val FRAME_SIZE = 1024

fun main()

fun main() {
    val settings = AudioSettings(samplingRate = SAMPLING_RATE, frameSize = FRAME_SIZE)

    println("Steam Audio ${SteamAudio.VERSION} - binaural quality probe")
    println("  ${settings.samplingRate} Hz, ${settings.frameSize}-sample blocks")

    SteamAudioContext().use { context ->
        Arena.ofShared().use { arena ->
            Hrtf.default(context, settings).use { hrtf ->
                interpolationSmoothness(context, settings, arena, hrtf)
                ambisonicOrderAccuracy(context, settings, arena, hrtf)
                spatialBlendCost(context, settings, arena, hrtf)
                summarise()
            }
        }
    }
}

private val findings = mutableListOf<String>()

Ear balance and interaural delay for one rendered impulse.

source — 43 lines, from line 63 
private class Cue(val ildDb: Double, val itdSamples: Int, val energy: Double)

private fun measureDirect(
    context: SteamAudioContext,
    settings: AudioSettings,
    arena: Arena,
    hrtf: Hrtf,
    binaural: BinauralEffect,
    params: BinauralEffectParams,
    input: AudioBuffer,
    output: AudioBuffer,
    azimuthDegrees: Double,
): Cue {
    val radians = azimuthDegrees * PI / 180.0
    binaural.reset()
    params.setDirection(sin(radians).toFloat(), 0f, (-cos(radians)).toFloat())

    // Steam Audio ramps parameter changes across a block, so an impulse at sample 0 would be
    // rendered with the previous direction. A silent block lets the ramp reach the target.
    input.clear()
    output.clear()
    binaural.apply(params, input, output)

    input.clear()
    input[0, 0] = 1f
    output.clear()
    binaural.apply(params, input, output)
    return cueOf(output, settings)
}

private fun cueOf(output: AudioBuffer, settings: AudioSettings): Cue {
    val left = FloatArray(settings.frameSize)
    val right = FloatArray(settings.frameSize)
    output.readChannel(0, left)
    output.readChannel(1, right)
    val peakL = left.maxOf { abs(it) }.toDouble()
    val peakR = right.maxOf { abs(it) }.toDouble()
    val ild = 20.0 * log10((peakR + 1e-12) / (peakL + 1e-12))
    val energy = sqrt(
        left.sumOf { it.toDouble() * it } + right.sumOf { it.toDouble() * it },
    )
    return Cue(ild, interauralDelay(left, right), energy)
}

Interaural delay by cross-correlation, in samples; positive means the right ear leads.

Cross-correlation rather than onset thresholding: bilinear HRTF interpolation sums two measured responses with different delays, so both ears show early energy and a fraction-of-peak onset detector collapses the delay to zero. That is a measurement artefact, not an absence of interaural delay. Cross-correlation over a broadband impulse is unambiguous, unlike over a periodic signal.

private fun interauralDelay(left: FloatArray, right: FloatArray, maxLag: Int = 64): Int {
    // Restrict to the region around the response so the search is cheap and not dominated by
    // the silent tail.
    val start = (left.indices.firstOrNull { abs(left[it]) > 1e-4f } ?: 0).coerceAtLeast(maxLag)
    val end = minOf(left.size - maxLag, start + 512)
    if (end <= start) return 0

    var bestLag = 0
    var bestScore = -Double.MAX_VALUE
    for (lag in -maxLag..maxLag) {
        var sum = 0.0
        for (index in start until end) sum += left[index].toDouble() * right[index + lag]
        if (sum > bestScore) {
            bestScore = sum
            bestLag = lag
        }
    }
    // left[i] best matches right[i + lag]: lag > 0 means the right ear's copy occurs later in
    // its own array, i.e. the right ear is DELAYED, i.e. the source is on the left. Negate so
    // that positive means the right ear leads.
    return -bestLag
}

Question 1: interpolation smoothness across a finely swept azimuth.

source — 61 lines, from line 140 
private fun interpolationSmoothness(
    context: SteamAudioContext,
    settings: AudioSettings,
    arena: Arena,
    hrtf: Hrtf,
) {
    println()
    println("=".repeat(76))
    println("1. HRTF interpolation: how smoothly does the image move?")
    println("=".repeat(76))
    println()
    println("  Sweeping azimuth in 2-degree steps and measuring the ear-balance jump between")
    println("  adjacent steps. Large jumps are an audible zipper as a source moves.")
    println()

    BinauralEffect(context, settings, hrtf).use { binaural ->
        val input = AudioBuffer(arena, 1, settings.frameSize)
        val output = AudioBuffer(arena, 2, settings.frameSize)

        println("  interpolation   max step (dB)   mean step (dB)   distinct ILD values")
        println("  " + "-".repeat(66))

        val results = mutableMapOf<HrtfInterpolation, Pair<Double, Double>>()

        for (interpolation in HrtfInterpolation.entries) {
            val params = BinauralEffectParams(arena, hrtf, interpolation = interpolation)
            val ilds = (0..90 step 2).map { degrees ->
                measureDirect(
                    context, settings, arena, hrtf, binaural, params, input, output,
                    degrees.toDouble(),
                ).ildDb
            }
            val steps = ilds.zipWithNext { a, b -> abs(b - a) }
            val maxStep = steps.max()
            val meanStep = steps.average()
            // Round to 0.01 dB before counting: identical values mean the renderer reused the
            // same measured direction rather than interpolating between two.
            val distinct = ilds.map { Math.round(it * 100) }.distinct().size
            results[interpolation] = maxStep to meanStep
            println(
                "  %-15s %13.2f %16.3f %21d".format(
                    interpolation.name, maxStep, meanStep, distinct,
                ),
            )
        }

        val nearest = results.getValue(HrtfInterpolation.NEAREST)
        val bilinear = results.getValue(HrtfInterpolation.BILINEAR)
        findings += if (bilinear.first < nearest.first) {
            "BILINEAR interpolation cuts the worst ear-balance jump from " +
                "%.2f dB to %.2f dB -- use it for anything that moves.".format(
                    nearest.first, bilinear.first,
                )
        } else {
            "Interpolation mode made little measurable difference to smoothness here " +
                "(NEAREST %.2f dB vs BILINEAR %.2f dB worst step).".format(
                    nearest.first, bilinear.first,
                )
        }
    }
}

Question 2: how accurately does an Ambisonic round trip preserve direction?

source — 107 lines, from line 203 
private fun ambisonicOrderAccuracy(
    context: SteamAudioContext,
    settings: AudioSettings,
    arena: Arena,
    hrtf: Hrtf,
) {
    println()
    println("=".repeat(76))
    println("2. Ambisonic order: how much direction survives encode -> binaural decode?")
    println("=".repeat(76))
    println()
    println("  Reflections and pathing arrive as Ambisonics and are decoded binaurally, so")
    println("  their spatial sharpness is capped by the order. Direct binaural rendering is")
    println("  the reference -- the best the HRTF can do -- and each order is scored against it.")
    println()

    val azimuths = (0 until 360 step 15).map { it.toDouble() }

    // Reference: straight through a BinauralEffect.
    val reference = BinauralEffect(context, settings, hrtf).use { binaural ->
        val input = AudioBuffer(arena, 1, settings.frameSize)
        val output = AudioBuffer(arena, 2, settings.frameSize)
        val params = BinauralEffectParams(arena, hrtf, interpolation = HrtfInterpolation.BILINEAR)
        azimuths.associateWith { azimuth ->
            measureDirect(context, settings, arena, hrtf, binaural, params, input, output, azimuth)
        }
    }

    println("  Note: ILD is the reliable measure here. Interaural DELAY is not meaningful for")
    println("  an Ambisonic decode -- the field is resynthesised from spherical harmonics, so")
    println("  the fine time structure is not preserved the way level is. It is shown for")
    println("  completeness, not as a quality score.")
    println()
    println("  order  channels   ILD error (dB)   ITD delta (samples)   peak |ILD| vs ref")
    println("  " + "-".repeat(74))

    val referencePeak = reference.values.maxOf { abs(it.ildDb) }
    val orderScores = mutableMapOf<Int, Double>()
    val orderPeaks = mutableMapOf<Int, Double>()

    for (order in 1..3) {
        val channels = SteamAudio.ambisonicChannelCount(order)
        AmbisonicsEncodeEffect(context, settings, maxOrder = order).use { encode ->
            AmbisonicsDecodeEffect(context, settings, hrtf, maxOrder = order).use { decode ->
                val mono = AudioBuffer(arena, 1, settings.frameSize)
                val ambisonic = AudioBuffer(arena, channels, settings.frameSize)
                val stereo = AudioBuffer(arena, 2, settings.frameSize)
                val encodeParams = AmbisonicsEncodeEffectParams(arena, order)
                val decodeParams = AmbisonicsDecodeEffectParams(
                    arena, order, binaural = true, hrtf = hrtf,
                )

                var ildSquared = 0.0
                var itdSquared = 0.0
                var peak = 0.0

                for (azimuth in azimuths) {
                    val radians = azimuth * PI / 180.0
                    encode.reset()
                    decode.reset()
                    encodeParams.setDirection(sin(radians).toFloat(), 0f, (-cos(radians)).toFloat())

                    // Settle both stages before the impulse; see measureDirect.
                    mono.clear(); ambisonic.clear(); stereo.clear()
                    encode.apply(encodeParams, mono, ambisonic)
                    decode.apply(decodeParams, ambisonic, stereo)

                    mono.clear()
                    mono[0, 0] = 1f
                    ambisonic.clear()
                    stereo.clear()
                    encode.apply(encodeParams, mono, ambisonic)
                    decode.apply(decodeParams, ambisonic, stereo)

                    val cue = cueOf(stereo, settings)
                    val ref = reference.getValue(azimuth)
                    ildSquared += (cue.ildDb - ref.ildDb) * (cue.ildDb - ref.ildDb)
                    itdSquared += ((cue.itdSamples - ref.itdSamples).toDouble()).let { it * it }
                    peak = maxOf(peak, abs(cue.ildDb))
                }

                val ildError = sqrt(ildSquared / azimuths.size)
                val itdError = sqrt(itdSquared / azimuths.size)
                orderScores[order] = ildError
                orderPeaks[order] = peak
                println(
                    "  %5d %9d %16.2f %21.1f %18.1f / %.1f".format(
                        order, channels, ildError, itdError, peak, referencePeak,
                    ),
                )
            }
        }
    }

    val one = orderScores.getValue(1)
    val two = orderScores.getValue(2)
    val three = orderScores.getValue(3)
    findings += ("Use Ambisonic order 2 for the reflected and pathed buses. Order 1 reaches only " +
        "%.0f%% of direct binaural's peak ear-balance and carries %.1f dB RMS error; order 2 " +
        "reaches %.0f%% for %.1f dB. Order 1 -> 2 removes %.0f%% of the error for 9 channels " +
        "instead of 4, whereas 2 -> 3 removes only a further %.0f%% and costs 16.").format(
        orderPeaks.getValue(1) / referencePeak * 100, one,
        orderPeaks.getValue(2) / referencePeak * 100, two,
        (one - two) / one * 100,
        (two - three) / two * 100,
    )
}

Question 3: what does pulling a source toward the head cost?

source — 56 lines, from line 312 
private fun spatialBlendCost(
    context: SteamAudioContext,
    settings: AudioSettings,
    arena: Arena,
    hrtf: Hrtf,
) {
    println()
    println("=".repeat(76))
    println("3. Spatial blend: what does un-spatialising cost?")
    println("=".repeat(76))
    println()

    BinauralEffect(context, settings, hrtf).use { binaural ->
        val input = AudioBuffer(arena, 1, settings.frameSize)
        val output = AudioBuffer(arena, 2, settings.frameSize)

        println("  spatialBlend   |ILD| at 90 deg   fraction of full spatialisation")
        println("  " + "-".repeat(64))
        var full = 0.0
        for (blend in listOf(1.0f, 0.75f, 0.5f, 0.25f, 0.0f)) {
            val params = BinauralEffectParams(
                arena, hrtf,
                interpolation = HrtfInterpolation.BILINEAR,
                spatialBlend = blend,
            )
            val cue = measureDirect(
                context, settings, arena, hrtf, binaural, params, input, output, 90.0,
            )
            val ild = abs(cue.ildDb)
            if (blend == 1.0f) full = ild
            println(
                "  %12.2f %17.1f %31.0f%%".format(blend, ild, if (full > 0) ild / full * 100 else 0.0),
            )
        }
    }
    findings += "spatialBlend below 1.0 trades localisation for a closer, more diffuse image; " +
        "keep it at 1.0 for anything the player should be able to place."
}

private fun summarise() {
    println()
    println("=".repeat(76))
    println("Recommended settings for best binaural presentation")
    println("=".repeat(76))
    println()
    findings.forEach { println("  - $it") }
    println()
    println("  - Share one HRTF across every effect. It is ~15 MB of native memory (measured),")
    println("    so one per listener, never one per source.")
    println("  - Keep the direct path on a BinauralEffect rather than folding it into the")
    println("    Ambisonic bus: direct binaural is the accuracy ceiling, and order-1 Ambisonics")
    println("    throws away a measurable part of it.")
    println("  - Never binauralise twice. Decoding an Ambisonic bus with binaural = true is")
    println("    already an HRTF render; putting a BinauralEffect after it doubles the")
    println("    head-shadowing and smears the image.")
}