How the sound is shaped
This page explains, with graphs, how CarDSPMusicPlayer changes the sound with its equalizer and bass/treble controls. Every graph is drawn live from the same formulas the app actually uses. Move the sliders and see how the response changes.
Everything on this page applies to both the iPhone and Android versions. For features found in only one version (such as in-car acoustic correction on iPhone, or loudness normalization and silence trimming on Android), see each version's support page.
1Signal flow
On its way to your speakers or earphones, the audio is processed in this order.
- Song data
- 28-band equalizerRaises or lowers the level at 28 individual frequencies
- Bass / trebleRaises or lowers the whole low end or the whole high end at once
- Auto trimIf you boost too much, automatically lowers the overall level a little
- LimiterThe final safety net. Holds peaks down just before they would clip (always active)
- Speakers / earphones
Turning the equalizer off bypasses the equalizer, bass/treble and auto trim together. The limiter is always active. The iPhone version always lowers the level by 6 dB at the input to create headroom before processing.
2Pitch and "frequency"
Sound is a vibration of the air. The number of vibrations per second is called the frequency (Hz, hertz): the fewer, the lower the sound; the more, the higher. Human hearing covers roughly 20 Hz to 20,000 Hz (20 kHz).
Doubling the frequency raises the pitch by one octave (from one C on the piano to the next). That's why the horizontal axis of the graphs on this page is logarithmic — spaced by doublings rather than evenly.
3Stacked, not split
When you hear "equalizer", you might imagine the sound being split into 28 pieces, each adjusted in level and then mixed back together. A car audio crossover (channel divider) really does work that way, sending the lows to the woofer and the highs to the tweeter.
This app's equalizer does not split the sound. It passes a single signal through 28 filters in series. Each filter changes only the region around its own frequency and lets almost everything else through untouched — like shining light through 28 stacked sheets of tinted, transparent film.
- No "seams" from remixing — phase misalignment or dips at the crossover points are much less likely
- With every band at 0 dB (Flat), the filters do nothing at all, and the original sound passes through unchanged
4Each filter is a "bell"
All 28 filters are bell-shaped (technically, peaking filters). The change is greatest at the filter's own center frequency and tapers smoothly back to 0 dB the farther you move away from it. A boost makes a peak; a cut makes a dip.
The change at each frequency when only this band is moved. The bell width (Q) is 4.3 for every band. Each band can be adjusted by ±8.5 dB.
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5Stacking 28 bells
The 28 bands are spaced at 1/3-octave intervals (three per octave) from 31.5 Hz to 16 kHz. Neighboring bells are wide enough for their skirts to overlap, so the 28 stacked together form one continuous curve.
The thin lines are the 28 individual bells; the thick line is the result of stacking them all. The example curves are for illustration and differ from the app's presets.
Look closely and you'll see a small ripple in the combined result: the response sags slightly between bells, and neighboring skirts add up. For example, setting every band to +3 dB actually gives about +3.8 to +4.9 dB (a ripple of about 1 dB). This is normal behavior for a design that prioritizes the precision of each individual band.
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6Bass / treble are "shelves"
Bass and treble are not bells but shelving filters. Beyond a corner frequency, they raise or lower everything in one go. It's the same idea as the BASS and TREBLE knobs on a classic amplifier. Use the 28 bands to shape the details, and bass/treble to set the overall tilt.
Bass lifts (or lowers) everything below 150 Hz, and treble everything above 4 kHz, like a flat shelf. Both range ±8.5 dB. Used together with the 28 bands, the effects add up.
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7A feel for "dB" (decibels)
Level changes are expressed in dB. They're convenient because you can simply add them, but the actual amplitude changes by multiplication.
| Change | Amplitude | Perceived loudness (approx.) |
|---|---|---|
| +1dB | about 1.12× | Barely noticeable |
| +3dB | about 1.4× | A little louder |
| +6dB | about 2× | Clearly louder |
| +8.5dB | about 2.7× | Much louder (the per-band maximum) |
| +10dB | about 3.2× | Sounds about twice as loud |
| −6dB | about 0.5× | Clearly quieter |
8"Auto trim" prevents over-boosting
Digital audio has a ceiling (0 dBFS) beyond which nothing can be represented. Anything above it is chopped off, producing harsh clipping distortion. Many recent recordings are already mastered close to that ceiling, so a large EQ boost pushes them over it very quickly.
So the app automatically lowers the overall level by just enough whenever the highest boost would exceed +9 dB. The highest boost is calculated including any overlap with bass/treble. Lowering the overall level does not change the shape of the curve.
The dashed line is the curve as adjusted; the solid line is the curve after auto trim has lowered the overall level. The reduction is at most 18 dB, and it never raises the level.
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9The final safety net: a look-ahead limiter
Auto trim is calculated from the shape of the curve, so it can't catch momentary peaks in the music (such as drum attacks). That's why a limiter is always running at the very end. It looks 3 ms ahead and starts reducing the gain smoothly just before a loud peak arrives.
Using a sound like two drum hits as an example, this compares the level before and after the limiter (peak per millisecond). Only the parts that would exceed −2 dB are reduced, and the output never exceeds −1 dB. The output is delayed by the look-ahead (3 ms), but the two are aligned here for easy comparison.
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| Parameter | Value | Meaning |
|---|---|---|
| Threshold | −2dB | Only peaks that would exceed this are smoothly reduced |
| Ceiling | −1dB | Nothing is output above this level |
| Look-ahead | 3 ms | Prepares before a loud peak arrives |
| Attack | 1 ms | Reacts quickly to loud peaks |
| Release | 100 ms | Recovers slowly so the level doesn't pump unnaturally |
With tracks that are mastered right up to the ceiling (much recent pop and some live albums), the limiter works continuously, so the sound may seem slightly flattened or compressed. This comes from how the track was mastered — it is not a fault.
10Summary
| Stage | What it does | Key values | When active |
|---|---|---|---|
| 28-band equalizer | Bell-shaped boost/cut at 28 frequencies | 31.5 Hz–16 kHz, 1/3 octave, Q = 4.3, ±8.5 dB | When the EQ is on |
| Bass | Boosts/cuts the low end as a whole | Below 150 Hz, ±8.5 dB | When the EQ is on |
| Treble | Boosts/cuts the high end as a whole | Above 4 kHz, ±8.5 dB | When the EQ is on |
| Auto trim | Lowers the overall level if boosted too much | By the amount above +9 dB, up to −18 dB | When the EQ is on |
| Limiter | Holds down momentary peaks | Threshold −2 dB, ceiling −1 dB, 3 ms look-ahead | Always |
All processing is done digitally in 32-bit floating point. Because no analog circuitry (resistors, capacitors) is involved, there is no component tolerance or temperature drift — the result is always exactly the same.