489 Audio Transient Shaper :: Quicker, easier and cheaper to make your own chip!

How it works

This transient shaper processes audio signals in real-time by independently controlling the attack (initial transient) and sustain (decay) portions of the signal using dual envelope followers.

Architecture

The design implements three main components:

Fast Envelope Follower: Tracks rapid changes in the audio signal with a time constant of 0.25 (attack detection)
Slow Envelope Follower: Tracks gradual changes with a time constant of 0.125 (sustain detection)
Modulation Stage: Applies independent gain control to attack and sustain components

Signal Flow

6-bit audio input is fed to both envelope followers simultaneously
Fast envelope captures transient peaks for attack enhancement
Slow envelope tracks sustained signal energy
Each envelope can be boosted by 50% when enabled via control bits
Final output combines original signal with envelope-derived gain modulation

Processing Parameters

Clock frequency: 24 MHz
Attack time constant: 0.25 (fast response)
Sustain time constant: 0.125 (slow response)
Boost amount: 50% (right-shift by 1)
Bit depth: 6-bit input, 8-bit output

How to test

Input Configuration

The 8-bit input (ui_in) is mapped as follows:

Bits [5:0]: Audio input (6-bit unsigned, 0-63 range)
Bit [6]: Sustain amount (0=bypass, 1=boost sustain)
Bit [7]: Attack amount (0=bypass, 1=boost attack)

Test Procedure

Passthrough Test: Set ui_in = 0b00XXXXXX (attack/sustain off) and verify output equals input
Attack Enhancement: Feed an impulse signal (high→low transition) with ui_in[7]=1 to see transient boost
Sustain Control: Apply a constant signal with ui_in[6]=1 to verify sustained level boost
Combined Mode: Enable both bits for maximum dynamic processing

Example Test Vectors

ui_in = 0b00100000 → Passthrough (audio=32, no boost)
ui_in = 0b10100000 → Attack boost enabled (audio=32)
ui_in = 0b01100000 → Sustain boost enabled (audio=32)
ui_in = 0b11111111 → Full boost (audio=63, both enabled)

Clock & Reset

Apply 24 MHz clock to clk
Assert rst_n low for at least 10 cycles to reset
Set ena high to enable processing

External hardware

This design operates entirely in the digital domain and does not require external hardware. For practical audio applications, you would need:

ADC: To convert analog audio input to 6-bit digital (e.g., parallel flash ADC)
DAC: To convert 8-bit digital output back to analog audio (e.g., R-2R ladder)
Anti-aliasing filter: Input low-pass filter for frequencies above ~1 MHz
Reconstruction filter: Output low-pass filter to smooth the digital-to-analog conversion

Optional but recommended:

Input level control (potentiometer + voltage divider)
Output amplifier/buffer for driving speakers or headphones

#	Input	Output
0	audio_in[0]	audio_out[0]
1	audio_in[1]	audio_out[1]
2	audio_in[2]	audio_out[2]
3	audio_in[3]	audio_out[3]
4	audio_in[4]	audio_out[4]
5	audio_in[5]	audio_out[5]
6	sustain_amt	audio_out[6]
7	attack_amt	audio_out[7]

489 Audio Transient Shaper

489 : Audio Transient Shaper