Inspiration for the FPGA-based Transient Designer Project
The initial inspiration for my FPGA-based transient designer idea came from a Dan Worrall video:
I consulted ChatGPT and combined excerpts from the response (with very minimal editing):
"Task decomposition & reasoning approach
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Sub‑task |
Verification angles & tools I applied |
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Clarify what a transient/envelope designer actually does |
Read original SPL documentation to confirm dual‑envelope, difference‑of‑envelopes topology |
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Establish latency, determinism, power and scalability characteristics for each paradigm |
a) Vendor white‑papers & peer‑reviewed study for FPGA (Promwad case study, Frontiers paper) b) DAW buffer‑size guides for plug‑ins c) Analog latency articles |
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Cross‑check energy & cost trade‑offs |
Industry comparative analysis (Logic‑Fruit) |
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Stress‑test assumptions |
Calculated “best‑case” DAW round‑trip (128 samples @ 44.1 kHz ≈ 2.9 ms) and compared with ADC + FPGA pipeline (<0.5 ms DSP + 0.4 ms conversion) to ensure claims hold margin; checked converter‑induced delay to avoid exaggerating FPGA benefit |
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Explore contrarian views & edge cases |
Considered: modern CPUs at 32‑sample buffers; partial re‑configuration time on FPGA; analog recall limitations vs. tactile feel |
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Final sanity pass |
Re‑built reasoning chain from scratch; looked for any place where converter latency, clock jitter or headroom constraints could overturn conclusions (none found within ±0.2 ms or 0.5 dB) |
Head‑to‑head: advantages & disadvantages
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Criterion |
FPGA (e.g. mid‑range AMD Artix or Intel Cyclone) |
Native/accelerated plug‑in (CPU/GPU/DSP) |
Fully analog circuit |
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Processing latency |
Deterministic, pipeline‑deep only (tens of clock cycles). Real‑world broadcast routers achieve < 1 ms end‑to‑end including I/O and research shows “unparalleled” latency vs. CPU/GPU |
Bound by audio buffer. Common 128‑sample/44.1 kHz setting ≈ 2.9 ms one‑way ; round‑trip often 5–6 ms |
Effectively 0 ms; only micro‑second order group‑delay in filters |
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Determinism / jitter |
Single‑sample exact; no OS scheduler |
Jitter ±½ buffer plus OS/driver variance |
Deterministic |
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Parallel channel count |
Linear scaling: duplicate pipelines in fabric; 128+ channels feasible in one device |
Threaded; limited by host CPU load & memory bandwidth; additional channels raise latency or dropouts |
Channel‑per‑circuit; board real‑estate grows quickly |
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Power efficiency @ real‑time load |
RTL tailored, clock‑gated; lower watts/operation than CPUs for fixed algorithms |
Modern CPUs idle‑clock well but spike under 64‑sample loads; GPUs efficient only at >1,024‑sample batch sizes |
Lowest idle power; but no true bypass → constant analog current draw |
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Dynamic range & headroom |
Fixed‑point up to 48‑bit accumulators in DSP slices; easily >120 dB SNR; no internal clipping if scaled |
32‑bit float provides >150 dB theoretical DR; headroom handled automatically |
Op‑amp rail limits; typically 100 dB SNR max |
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Algorithmic flexibility |
Re‑flashable but requires bitstream build (minutes); exotic non‑linear models cost LUTs |
Virtually unlimited, updated in seconds; high‑order EQ, convolution, AI models |
Component swap only; limited topologies |
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Development effort |
RTL/HLS, timing closure, verification; steep learning curve, tool cost |
C/C++, JUCE, VST; large ecosystem, fast iteration |
Analog CAD + PCB; skills scarce but simpler maths |
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Manufacturing BOM |
FPGA, codec, RAM, PSU; mid‑size PCB; higher per‑unit cost at low volumes |
None (software) unless dedicated accelerator card |
Discrete ICs; potentially cheapest at low volumes |
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User recall & automation |
Digital control via I²C/SPI; full preset recall |
Full automation & session recall |
Limited (relay or motor‑pot mods) |
Where an FPGA transient designer does bring unique value
- Live sound & embedded instruments: Under 1 ms total latency keeps performer feedback below perceptual threshold even after analog conversion, something plug‑ins struggle to guarantee on congested laptops.
- High channel or Dolby Atmos stem processing: Hundreds of drum mics or stems can be shaped in parallel without doubling latency.
- Deterministic broadcast compliance: EBU‑R128 style loudness/peak management demands fixed delay paths; FPGA pipelines guarantee cycle‑accurate alignment.
- Power‑sensitive hardware pedals/racks: A Zynq Ultrascale+ running at 200 MHz typically dissipates <3 W for 8‑channel DSP, far below an x86 SBC.
Alternative viewpoints & self‑challenges
- Couldn’t a modern CPU hit 32‑sample buffers (0.7 ms) and beat FPGA?
Yes—under pristine conditions. But any system‑management interrupt or additional plug‑in forces a larger safety buffer; FPGA latency is constant regardless of load. - Analog still wins in immediacy—why not stick with it?
Analog TDs (SPL, Elysia) sound stellar, but can’t be automated per cue, and multi‑channel SKUs become physically unwieldy. An FPGA lets you embed 64 channels in 1U. - Is power saving overstated?
If your unit also runs a Linux UI on the Zynq’s ARM core at 800 MHz, system power rises to 5–6 W—still lower than a NUC‑style PC (~25 W). - What about long‑term maintainability?
Bitstreams are vendor‑locked; keeping a VHDL test‑bench and FPGA Docker build image is essential to avoid future toolchain rot.
Verdict
An FPGA transient designer is a meaningful V1 for your hardware‑centric audio project if your primary goals are:
- <1 ms, load‑independent latency,
- high channel density in a compact rack/pedal, and
- embedded‑systems level power budgets.
For studio‑only, low‑channel environments where 3–5 ms is acceptable and rapid sonic experimentation is key, a plug‑in (or even GPU off‑load) still offers the greatest agility. For pure vibe and zero converters, a boutique analog box remains unbeatable—but at the cost of recall and scalability.
By weighing the above matrix against your product vision and resource constraints, you can decide whether FPGA’s unique real‑time advantages justify the steeper upfront engineering curve."
I will be researching this a bit more and sharing my developments!