Pitch-shifting a recording, and where it breaks
EV·ENGINE has three recorded engines: the Ferrari 458, the BAC Mono and the Procar M1. Each recording covers a band of rpm rather than the whole rev range, and outside that band the audio is pitched to fit. At idle, the Ferrari sits 2.56 octaves below its lowest sample.
What the recorded band covers
The three sampled presets, the Ferrari 458, the BAC Mono and the Procar M1, each carry a single audio clip captured across one span of engine speed. The Ferrari's clip sits between 5,300 and 7,700 rpm. The BAC Mono's runs from 3,000 to 6,500 rpm. The Procar's is widest, from 3,200 to 8,000 rpm. Play any rpm inside that span and the recording is doing the work unmodified; play anything outside it and the same clip has to be pitched to reach the note.
Measured against each engine's full range from idle to redline, that band covers a different share for each car. The Ferrari idles at 900 rpm and redlines at 8,900, a span of 8,000 rpm, of which the recorded band accounts for 2,400: 30 percent. The BAC Mono idles at 1,000 and redlines at 9,000, also an 8,000 rpm span, with 3,500 of it inside the band: 44 percent. The Procar idles at 1,100 and redlines at 9,000, a 7,900 rpm span, of which 4,800 sits inside the band: 61 percent.
The Ferrari, the most exotic of the three recordings, is also the one that spends the least time as itself. Below 5,300 rpm and above 7,700 rpm, which together make up seventy percent of its rev range, the clip is stretched or compressed to reach the requested note. The Procar's straight six, by contrast, has the shallowest reach outside its own recording: its band runs to within a fifth of an octave of redline.
Below the band: pitching down to idle
Pitching a recording down slows it: a clip played back at half speed drops an octave, and every transient inside it, the mechanical clatter, the intake rasp, stretches to match. The distance from idle down to the bottom of the band is what decides how much stretching an idle note carries. The Ferrari's idle, at 900 rpm, sits 2.56 octaves below its lowest sample at 5,300 rpm. The BAC Mono's idle sits 1.58 octaves below its band at 3,000 rpm, and the Procar's 1.54 octaves below its band at 3,200 rpm.
Two and a half octaves of pitch-down is substantial: each octave halves the playback speed, so 2.56 octaves down means the Ferrari plays its recorded clatter at roughly a sixth of its true speed while the fundamental note still lands correctly. Listen for it against the reference four-cylinder, which is synthesised at every rpm and has no recording to bend: the Ferrari's idle has a looser, slightly smeared texture where the four-cylinder's stays tight.
The BAC Mono and the Procar, with lower band floors, need less than two-thirds as much pitch-down to reach their own idle. The difference is audible rather than dramatic: both still soften at idle, but neither carries the same stretched clatter the Ferrari does. Of the three, the Procar asks the least of its recording at the bottom, which is also where its band starts closest to idle.
Above the band: redline barely overshoots
At the top, none of the three recordings is stretched by nearly as much. The Ferrari's redline of 8,900 rpm sits 0.21 octaves above its band's ceiling of 7,700. The BAC Mono's 9,000 rpm redline sits 0.47 octaves above its band's 6,500 rpm ceiling, the largest of the three but still under half an octave. The Procar's redline sits only 0.17 octaves above its own band's 8,000 rpm ceiling.
All three cars were recorded being driven hard, which is why the top of each band already sits close to redline. The gap that remains is small enough that the pitch shift above it barely registers: no stretched clatter, a slightly thinner top few hundred rpm before the rev limiter would arrive in the real car.
Both the BAC Mono and the Procar carry an upshift point at 5,000 rpm, the rpm at which the gearbox shifts when pressed on rather than cruised, and that point sits inside both recorded bands. Most hard driving in either car stays inside the recording; the pitch-shifted final few hundred rpm near redline is the exception, not the rule.
Why the ten-second previews roll
Each engine's preview clip is rendered as a single sweep from idle to redline, stepped through an OfflineAudioContext rather than played live. For a synthesised preset that sweep is smooth throughout, because the engine is generated correctly at every rpm. For the three recorded presets, the same sweep has to cross into and back out of a fixed band, and the pitch shift either side of it is audible as the clip runs.
On the Ferrari's preview, the first stretch of the sweep sits as much as 2.56 octaves below the recording before the note snaps into its true register around 5,300 rpm, holds there for the width of the band, then climbs a further 0.21 octaves past it near redline. The BAC Mono and Procar previews roll less, because their idle-to-band gaps are smaller and their bands wider, but the seam where the pitched section meets the true recording is there to hear in all three.
It runs opposite to what the share figures suggest at first glance: a wider band, as on the Procar, means less of the preview is pitched material, so the join is crossed once early and once late rather than dominating the clip, while the Ferrari's narrower band means the true recording is a short island inside a longer stretch of pitched audio either side of it.
Recorded engines earn their texture inside the band and borrow it outside, which is a different trade to the one a synthesised preset makes. Nothing here is a defect in either approach: it reflects what recording one span of a real engine, rather than modelling every rpm, costs and buys.
Hear it
Ten seconds each: an acceleration run through the gears, then the overrun. The same presets the numbers above come from.
- FERRARI 458recorded · 4.5 V8 · flat-plane
- BAC MONOrecorded · 2.5 I4 · 9000 rpm
- PROCAR M1recorded · 3.5 I6 · 9000 rpm