The Rational Audiophile

[m1ts0s]

The Signal Path of Shame

There is an argument to be had over whether hi-fi’s primary limitation is its reliance on plain two-channel stereo. For the foreseeable future, however, it looks as though plain stereo will remain dominant, so the job of the hi-fi system would appear to be simply to ensure that the two channels that the record producers have created for us make it into the room as close to intact as possible – and indeed when this is done properly it can sound pretty spectacular.

However, at every stage in the recording and replay process, distortions are added to the signal. It would seem fairly straightforward that we would like to minimise those distortions. But while audiophiles and the hi-fi industry may pay lip service to this idea they then simply rip it up and start from the position that pimped-up copies of technology from circa 1950 are the best way to achieve this. This ancient technology contributes quite a few distortions that modern technology would not. There is no prospect of fundamental improvements.

Here is an incomplete list of those distortions:

Vinyl Gramophone

Noise and distortion

• basic noise floor of -70dB (A-weighted) if we’re lucky – stylus scraping along a groove in plastic, vinyl has finite grain size
• pops and clicks: scratches and dust
• electrical hum and noise: cartridge produces a tiny signal, and high gain pre-amplification is needed
• rumble: bearings, motor
• warped records cause various problems
• stylus wear
• stylus contamination: dust, dirt, vinyl particles
• stylus misalignment – may vary as arm moves across record
• record wear
• record contamination: dust, dirt, vinyl particles
• fundamental limitations in linearity of vinyl cutting/replay system
• diameter loss: speed of groove decreases throughout LP, increasing noise and distortion and reducing upper frequency response
• pre-echo: adjacent groove modulation
• microphony: sound from speakers feeds back into the pickup
• Channel separation: varies with frequency and typically only 20-30 dB at maximum
• Record may be pressed towards end of life of the stamper, resulting in increased levels of various distortions

Arbitrary processing needed for vinyl mastering

• compression (raises the quietest sections in volume to make them audible above the background noise, reduces the loudest sections to economise on groove spacing)
• de-essing (reduce treble response for high amplitude, high frequency sounds)
• mixing stereo bass to mono (otherwise the needle jumps out of the groove)

Wow and flutter

• off-centre pressing
• motor speed, belt etc.

Inaccurate frequency response

• RIAA record and/or playback curves are often only approximate
• combination of factors above (arbitrary processing when mastering, diameter loss etc.)

Valve Amplifier

• With fashionable ‘retro’ topologies THD can be of the order 1%-10%
• limited output power
• transformer coupling at output (some valve amplifiers claim to be output transformerless ‘OTL’, but this may be a case of throwing the baby out with the bathwater..?)
  • distortion and limited frequency response
  • high output impedance
  • lack of cone damping
    • inaccurate frequency response
    • distortion
• microphony: all valves are microphonic to some extent and so sound from speakers can feed back into the amp
• constantly degrading performance as valves age

Two-way passive speakers

With the standard two-way passive speaker there seem to be so many ‘anomalies’ that it’s difficult to know how to categorise them. So just as a simple list:

• Lack of bass due to small bass/mid driver, compact enclosure – a major, unnatural discrepancy between the original signal and what emerges into the room.
• Lack of damping
  • Passive crossover adds impedance between amp and driver: speaker cones are not under precise control
• passive crossover is extra, awkward load for amplifier
  • wastes amplifier power
  • amp has to work harder
    • higher distortion
• inaccurate crossover
  • varies with power and temperature – system naturally becomes harsher at higher output volumes
  • EQ control is ‘blunt’
  • EQ non-adjustable for room/speaker combination and placement
  • acoustic summing of driver signals that, due to imperfect crossover, are not complementary. May measure fine with steady state sine waves, but causes distortion of transients and music waveforms
• phase response is not flat
  • colours the sound independently of apparently-flat frequency response
  • smears detail
  • results in a form of actual distortion with non-steady state waveforms
• intermodulation distortion and doppler distortion
  • mid frequencies ‘ride’ on top of large bass woofer displacements
  • mid frequency output power limited by driver’s ability to handle bass
• beaming
  • woofer doubles up as mid: large diameter cone becomes directional at top end, mismatched with tweeter that has wide dispersion at crossover frequency
• iron-cored inductor (if used) saturation
  • distortion
  • impairment of filtering ability at high power
• breakup, power handling
  • woofer and tweeter reproducing frequencies outside their comfort zones -> distortion
• bass reflex
  • introduces more time domain smearing
  • port produces distortion and noise
  • port efficiency decreases with output power – sound becomes harsher as volume increases
  • inverted, delayed signal mixed with direct signal
    • may measure fine with steady state sine waves but causes distortion of transients and music waveforms
  • unnaturally-rapid roll-off
    • cannot take advantage of room gain
    • effectively a deep hole in the response compared to what was recorded
  • uncontrolled cone below resonance
    • woofer still has to produce mid frequencies as cone flaps about

Clearly the ‘high end’ hi-fi system that most audiophiles aspire to own is anything but transparent and may actually be worse than conventional measurements suggest. It is possible that improvements in any one area may actually make the overall sound worse. For example, feeding a digital recording with wide bandwidth and high dynamic range into a valve amp / passive speaker system may reveal limitations in those components that the more gentle, compressed, vinyl version does not. (Avoidance of problems could also be ensured by consciously or unconsciously choosing anodyne music and recordings, but surely audiophiles would never tailor their taste in music to reflect the limitations of their systems… would they..?)

Randomly ‘trying things out’ cannot be expected to fix the problems. The rational approach is not to start from where we are now, nor to accept the numerous rules of thumb that the audio industry believes in (phase doesn’t matter, harmonic distortion is benign etc.) but simply to attempt to produce a genuinely transparent system using whatever tools are available in the 21st century.

https://therationalaudiophile.wordpress.com/

 

[m1ts0s]

The Signal Path of Virtue

Just to be contrary, I am working on a hypothesis that flies in the face of conventional hi-fi wisdom. I am asking the heretical question: what if sound quality, or “musicality”, might be related to objective specifications and design, regardless of price and the avant garde-ness of the components? What if a ‘virtuous’ system could be built from ordinary components connected together in a sensible way?

One such virtuous system, I would suggest, might comprise the following:

– Digital Source (CD, FLAC, DVD etc.)
– DSP crossover filtering and driver correction
– Solid state amplification (one amp per driver)
– Three-way sealed speakers

On the face of it, this would reduce, or provide mechanisms to address, all of the problems listed in The Signal Path of Shame.

But of course it cannot be so easy or we could not have the situation where audiophiles are falling over themselves to pay car-sized sums for delicate, maintenance-hungry turntables and valve amplifiers! Nevertheless I am giving it a go. At some point the problems will become apparent, I am sure…

Linear Phase Filters

One aspect that I have so far not been picked up on, and which might be suggested by some as a potential drawback in such a Signal Path of Virtue, is the sonic effect of linear phase filters – which are necessary if we wish to eliminate phase distortion in each speaker driver. This is a distinct difference between 1950s technology and the fully-implemented DSP system.

All filters ‘ring’ if fed with a step waveform but analogue filters only ‘post-ring’. It is suggested that the post-ringing is effectively masked by musical signals, which usually follow a sharp attack with a longer decay or tail.

In contrast, in order to avoid the phase shift introduced by conventional filters we use linear phase filters (which are further modified in order to get fully complementary acoustic outputs from the drivers). If a step is fed into a linear phase crossover filter it will ‘pre-ring’ and, it is suggested, this is not masked by the signal as effectively as ‘post-ringing’. However, unlike a conventional analogue crossover, the two systems (low pass and high pass) that constitute the crossovers and drivers will complement each other perfectly, and the ringing from each filter will acoustically sum with its opposite thus cancelling out to zero – although in practice there is bound to be some discrepancy. This discrepancy is cited by some people as a problem. It is likely that the discrepancy is exacerbated in a two-way speaker (as opposed to a three-way) by the large difference in dispersion angles between drivers at the crossover frequency (so another example of why the system needs to be designed ‘holistically‘).

A steeper filter will ring more and for longer, but the ‘problem’ (if that is what it is) is not significant with real music signals and the relatively gentle crossover filters that we use. There is also a perceptive pre-masking effect from a loud sound even if it occurs after a quiet sound which further reduces the theoretical problem.

Attempts to quantify the sonic effects of ringing in various linear phase crossover filters have been made, concluding that it is not an audible phenomenon when used in moderation (i.e. avoiding filter orders of greater than 600!! My system currently uses 4th order filters…).

If the audiophile feels that pre-ringing is unacceptable under any circumstances it is only necessary to modify the DSP filters to mimic conventional analogue filters, re-introducing phase shifts and relegating any ringing to the ‘post-‘ variety. However, in this case there would be no theoretical summing of the ringing to zero and the benefits of truly flat phase response would be lost.

 

[m1ts0s]

Does hi-fi end here?