What does Rob mean by 'starting and stopping of notes', timbre variation and 'sense of flow', and what causes them?

Rob Watts ties all three listening attributes to how accurately a DAC's interpolation filtering reconstructs the timing of transients, and names noise floor modulation as a second cause where timbre is concerned Head-Fi, Mar 2016Head-Fi, Mar 2020. Each term is taken in turn below, with his statements in date order.

Starting and stopping of notes

What he means. In a March 2016 blog post he lists "starting and stopping of notes" as one of his listening-test factors: "This is the ability to hear the starting of a note and its about the accuracy of transient timing." He continues that "Any uncertainty in timing will soften edges, making it difficult to perceive the initial crack from a woodblock, or all the keys being played on a piano" Head-Fi, Mar 2016. In April 2017 he said the stopping of notes is heard most easily on electronica, because "electronic instruments are unique in that the stopping can be immediate" Head-Fi, Apr 2017. In a Nintronics open day talk of November 2018 (uploaded December 2018, check the recording) he said that when he thinks of stopping of notes he is "thinking about electronica music" Pursuit Perfect System 23:08.

Hugo, 2013 to 2015: noticed first, explained in different terms. In October 2014 he said Hugo had "unique qualities - particularly the way it expresses the starting and stopping of notes", and that he was "profoundly disturbed intellectually" on first hearing the prototype: "where was this performance coming from? It turned out that it was a combination of factors, mostly due to the internal FPGA code upgrade I have been doing over the past 7 years" Head-Fi, Oct 2014. Twelve days later, on Hugo's first anniversary, he recalled the first listening test (October 2013): he "was not sure where this performance was coming from", but a year on he knew "the improved musicality was principally due to the re-writing of the code from the bottom up", plus the space the Xilinx Spartan-6 FPGA gave him Head-Fi, Oct 2014. In November 2015 he named a more specific cause (next section). In a June 2017 post he recalled that on first hearing Hugo he noticed "exceptional timing (being able to hear the starting and stopping of notes properly)" Head-Fi, Jun 2017. In October 2016 he wrote that the issue "is actually a relatively new attribute for me" and that he "only really appreciated how important it was with Hugo" Head-Fi, Oct 2016.

The cause as he gave it, 2015 to 2016. In a September 2015 post on DSD he said that DSD's microsecond timing non-linearity "is very audible and it affects the ability of the brain to perceive the starting and stopping of instruments" Head-Fi, Sep 2015. In November 2015 he said the source of Hugo's ability was not the WTA filter, because earlier 32,000-tap designs "did not sound like Hugo"; he credited much better 2048 FS interpolation filters, which he said he had used to improve noise floor modulation, and concluded "it was the time domain accuracy that was important" Head-Fi, Nov 2015. In that post the FPGA code upgrade ("a 6 year development") is the setting and the interpolation filters are the specific cause; in October 2014 he had credited the code rewrite and the larger FPGA more broadly. For Dave (October 2016) he said that adding a 256 FS WTA filter "allowed for a better appreciation of the starting and stopping of notes - and things sounded sharper, faster and tighter" Head-Fi, Oct 2016.

The 16 FS to 256 FS step, 2017 to 2021. In March 2017 he introduced the filter option on Hugo 2 and said "What you hear is an immediate change in the ability to perceive the starting and stopping of notes" Head-Fi, Mar 2017. In July 2019, speaking of his filters in general, he separated this from the first WTA stage (1 FS to 16 FS): "WTA1 we get big changes in bass, pitch, timbre and instrument separation - but WTA 2 only affects the perception of starting and stopping of notes" Head-Fi, Jul 2019. In September 2021 he said that with WTA filters above 16 FS the gains so far were "more limited to clarity, soundstage and the perception of starting and stopping of notes", in contrast to the M Scaler's gains in tempo, pitch, timbre variation and instrument separation Head-Fi, Sep 2021.

Other causes he named. In January 2020 he said DSD's amplitude-related delay "makes it difficult for the brain to perceive the starting and stopping of transients - so you can no longer hear the starting and stopping - that makes it sound soft" Head-Fi, Jan 2020. In July 2019 he said that with zero-order-hold ("NOS") filtering the timing errors are enormous, and that "if one can't perceive edges, then things sound unnaturally soft" Head-Fi, Jul 2019.

Still a test in 2026. In a podcast published October 2026 (recording date not stated) he listed perceiving "the starting and stopping of notes more accurately" among the signs that "you've made a fundamental improvement" Part-Time Audiophile 8:21.

Timbre variation

What he means. From the same 2016 post: timbre is "where bright instruments simultaneously sound bright together with rich and dark instruments - the rich and smooth tones of a sax should be present with the bright and sharp sound of a trumpet." He added that "Normally, variation in timbre is suppressed, so everything tends to sound the same" and named two factors: noise floor modulation, and "the accuracy of timing of transients" Head-Fi, Mar 2016. In April 2014 he described live music against hi-fi in the same terms: "Timbre is compressed - you don't get a really rich and smooth instrument playing at the same time as something bright" Head-Fi, Apr 2014. In August 2023 he gave a listening check: "play a track that has a lot of timbre variation (sax and trumpet say - does the sax sound warm and soft and does the trumpet sound sharp and hard at the same time)" Head-Fi, Aug 2023.

Cause 1: transient timing. In September 2015 he said the initial transient is used by the brain to determine an instrument's timbre, "and if timing of transients is non-linear, then we get compression in the perception of timbre" Head-Fi, Sep 2015. In November 2016, on the Dave thread, he said timbre variation was down to "Dave's complete lack of noise floor modulation, plus the unique way it accurately reconstructs the timing of transients" and that "when transient timing is distorted, one can't perceive timbre correctly" Head-Fi, Nov 2016. In a February 2018 seminar he cited psychoacoustic experiments in which the starting transients of a trumpet and a piano are removed and "listeners find it very difficult to tell the difference", concluding that "transients are really important for you to recognise the timbre of an instrument" MajorHiFi 5:43. He gave the same example in an interview about the M Scaler published in August 2022 (recording date not stated), adding that if transients "are being modulated by other instruments it becomes difficult for the brain to make sense of what's going on" and timbre perception is suppressed or degraded AVstore 32:29.

Cause 2: noise floor modulation. In June 2014, comparing optical and coaxial input on Hugo, he described noise floor modulation as noise that tracks the signal, so that a rich sax picks up a bright component the brain cannot separate from it: "We actually end up with all instruments sounding bright, so timbre variations are suppressed" Head-Fi, Jun 2014. In October 2021 he wrote that "any noise floor modulation, no matter how small, will change your perception of timbre" Head-Fi, Oct 2021. He repeated the sax example at CanJam NYC in March 2024: "So now your timbre of a saxophone is brighter than it should be" MFG Audio Reviews 10:02. In an interview published October 2026 (recording date not stated; its transcript places the recording before May 2026) he said that when noise floor modulation is added "the bright instruments become more etched, but your dark instruments also become brighter", which is how he says to spot a distortion HiFi Buys 48:54.

Other causes he named. Inductors and ferrites in the analog path, because phase shift that varies with signal level "will modulate the timing of transients" and degrade timbre variation (April 2021) Head-Fi, Apr 2021. Minimum-phase filters, which he said sound "flat, soft and ill defined, with very poor timbre variations and pitch reproduction" compared with a WTA filter (March 2018) Head-Fi, Mar 2018.

Variation as his yardstick. In April 2014 he said "if a modification makes the sound more variable, then its more expressive, and hence more transparent", even if it sounds darker or smoother Head-Fi, Apr 2014. In the M Scaler interview published in August 2022 he listed timbre variations among the things he listens for when tuning filters AVstore 18:29.

Sense of flow

Passing uses, 2016 to April 2018. The earliest item found that uses the word is an April 2016 post comparing Class D with digital reconstruction, where he said that with both "you can't hear the starting and stopping of notes correctly, and the music does not flow properly" Head-Fi, Apr 2016. In July 2016 he said of Dave's galvanically isolated USB input that "Detail resolution and flow is slightly better than optical" Head-Fi, Jul 2016. In April 2018, on the Hugo 2 thread, he said he suspected a new filter (not named in the post) "sounds smoother and warmer, with a much more natural sense of flow and rythmn" Head-Fi, Apr 2018. None of these posts defines the term. He later withdrew the July 2016 USB preference: in January 2018 he wrote "I used to think that USB had the edge on optical - it's slightly brighter - but I was wrong about that", putting the brightness down to "worse RF noise pick-up" Head-Fi, Jan 2018; in March 2022 he called the 2016 post one "where I made a mistake" Head-Fi, Mar 2022; and in October 2022 he said "there is no technical benefit to using USB over optical when using my DACs" Head-Fi, Oct 2022. In February 2019, between those, he still wrote "Technically USB is better from the clock POV; but in practice I have had optical sound identical to USB" Head-Fi, Feb 2019.

Hugo M Scaler, July to October 2018. In a July 2018 interview at CanJam London (published August 2018) he said someone he played the M Scaler to said it "sounds slower", and added, "I'd always appreciated the sense of flow as being different" Pursuit Perfect System 2:47. In the same interview: "Without the M Scaler, the flow of the music is mechanical and jerky, and it sounds disjointed", and with it "you can actually perceive instruments talking to one another" Pursuit Perfect System 3:35. (The passage describing the listener is garbled in the transcript. In a November 2018 post he called that listener a non-audiophile Head-Fi, Nov 2018.) In October 2018 he listed "timbre variation,tempo and flow, and instrument separation" as better with the Hugo M Scaler and TT2 than with Dave, while saying Dave is more transparent Head-Fi, Oct 2018. The same month, on the Hugo M Scaler thread, he said "the new DSD filter" "sounds warmer with enhanced sense of flow than the Dave DSD+ filter" Head-Fi, Oct 2018.

His explanation, November 2018 onward. He said he heard the M Scaler's effect as "a reduction in the dominance effect (where the loudest instrument draws one's attention) which means that your attention is constantly jerking from one instrument to another; another way of looking at this is to say the sense of flow is more natural; but this all adds up to the impression of a slower tempo" Head-Fi, Nov 2018. The technical reason he gave in that post is that with shorter filters "individual instruments in the past and future modulate the timing of transients making them slightly early or late" Head-Fi, Nov 2018. In May 2019 he said he had only recently characterized the effect as slower tempo: "Hence tempo sounded jerky or mechanical before the M scaler; with it, it flows much more naturally" Head-Fi, May 2019.

The fullest statement, March 2020. On the Hugo M Scaler thread he wrote: "Without it, things sound mechanical and disjointed". He said the disjointed quality, where attention shifts to the loudest instrument, "makes it sound artificially faster." He said the technical cause is that transient timing is "constantly being modulated by instruments in the immediate past and immediate future", which makes it harder for the brain to separate instruments, and that "this I think is the reason why the music then sounds mechanical and disjointed". He also said the M Scaler "doesn't eliminate this issue" Head-Fi, Mar 2020. He called the change in flow the principal reason listening fatigue is lower with the M Scaler, saying that if instruments are harder to separate "you get more listening fatigue" Head-Fi, Mar 2020.

Tempo is perceived, not changed. In August 2019, on the Qutest thread, he said "The perception of tempo is subjective" and that two effects pull in opposite directions: better transient perception raises perceived tempo, while less dominance lowers it. With an M Scaler on a Qutest, he said the biggest effect is on dominance, "so on balance it sounds slower and more measured, with a more natural sense of flow" Head-Fi, Aug 2019. He also said which effect wins "will depend upon the existing DAC, the rest of your system, and the bit between your ear lobes" Head-Fi, Aug 2019. In May 2021 he said "tempo for sure is not changing in an absolute sense, but your perception of tempo is changing" Head-Fi, May 2021. In January 2022 he described the dominance effect as creating "the impression of snapping backwards and forwards, which artificially spices up the tempo" Head-Fi, Jan 2022. In the HiFi Buys interview published October 2026 (recording date not stated; its transcript places the recording before May 2026) he kept both directions open: "Sometimes things can sound faster when you make an improvement. Sometimes it can sound slower when you're making an improvement", depending on the instrument and the music HiFi Buys 48:07.

Other things that move perceived tempo. In August 2018 he said reducing RF noise made things warmer and darker and that "more midrange gives the impression of a slower tempo" Head-Fi, Aug 2018. In June 2014, comparing DSD and CD on Hugo, he said "perception of tempo can change markedly depending upon distortions" Head-Fi, Jun 2014.

Flow claims for other products. In May 2019 he said the Hugo 2 DSD filter made DSD "warmer, and smoother and flowed much more naturally (I am talking about tempo here)" Head-Fi, May 2019. In December 2019 he described a Blu 2 with Dave at a show as having "a natural sense of flow" Head-Fi, Dec 2019. In December 2020 he said an M Scaler would add, among other things, a "more natural sense of flow" Head-Fi, Dec 2020. These are listening impressions; the posts give no separate technical explanation of flow.

Later restatements. In March 2023 he repeated the listener anecdote and said the M Scaler "sounded slower because you haven't got this mechanical jerkiness to the rhythm" Eric's Hi-Fi 38:17. In September 2025 he said "the tempo gets slower" with the M Scaler, "it loses that mechanical nature", and that "some people try it and say, I can't hear much of a difference" The Source AV 16:35The Source AV 16:52. In an interview published March 2026 (recording date not stated) he listed flow among the improvements from the Blackbird WTA interpolation filter, which he said in the same interview is only for the Quartet: "something that's difficult to explain, and that's the sense of flow" Audio Force 14:59Audio Force 16:18. He added that "the rhythm doesn't flow naturally" in digital music without it Audio Force 16:29. In an answer published by Soundnews in October 2026, asked what a listener notices first with the Quartet, he said "A huge amount depends upon the listener too" and listed "the natural sense of flow: tempo doesn't sound mechanical or jerky" article, Oct 2026.

What the three have in common

In talks from 2022 to 2025 he gives one sequence that covers starting and stopping and timbre variation (these passages do not mention flow). Interpolation filters leave transients slightly early or late, and "they're constantly being modulated by instruments in the past and in the future" Darko.Audio 10:12. The brain then cannot separate instruments, perceive timbre variation, define low frequency pitch, or perceive the starting and stopping of notes audio-2G 6:13. In 2024 he said the brain "uses the transient timing information to work out timbre, to work out pitch, to work out instruments where they're separated and to put instruments into space" The Speaker Shack 1:10. In a May 2025 presentation he listed rhythm and the sense of flow alongside tonal color, pitch, instrument separation and the starting and stopping of notes as factors he scores in listening tests Audiophile Junkie 12:30.

What he has not said

He describes flow mainly by its absence (mechanical, jerky or disjointed) and, when present, as music that "breathes" Head-Fi, Mar 2020 or as being able to perceive musicians' interplay Pursuit Perfect System 3:35The Source AV 16:52; he gives no fuller definition. He gives no measurement for flow or for timbre variation. He does not name the psychoacoustic experiments he cites on removing transients. Where he explains flow, he marks part of the reasoning as his own view ("I think") Head-Fi, Mar 2020. His technical explanation of flow is given for the Hugo M Scaler; for the other products where he mentions flow, this archive gives no separate explanation Head-Fi, May 2019Head-Fi, Dec 2020article, Oct 2026.

Quotes

“This is the ability to hear the starting of a note and its about the accuracy of transient timing.”2016-03-28 Head-Fi, Mar 2016

“Timbre perception comes from transients, and when transient timing is distorted, one can't perceive timbre correctly.”2016-11-10 Head-Fi, Nov 2016

“Hence tempo sounded jerky or mechanical before the M scaler; with it, it flows much more naturally.”2019-05-02 Head-Fi, May 2019

“tempo for sure is not changing in an absolute sense, but your perception of tempo is changing.”2021-05-04 Head-Fi, May 2021

“Then there is the natural sense of flow: tempo doesn't sound mechanical or jerky.”2026-10-17 article, Oct 2026

Dated positions

Caveats and open points

• Two passages are unclear in the transcripts: the garbled description of the listener in the July 2018 CanJam London interview (published August 2018) and the 'Blackbird WTA' passages in the March 2026 interview.
• He attributes the transient-removal findings (piano versus trumpet) to unnamed psychoacoustic experiments; this archive does not cite the studies.
• 'Flow' appears in passing from April 2016; his explanation of it dates from November 2018 onward, is tied to the Hugo M Scaler, is hedged ('I think'), and in an interview published March 2026 (recording date not stated) he called flow difficult to explain.
• No German or Japanese item is quoted.

Topics: Transients, rhythm and flow, Timbre, pitch and instrument separation, Transient timing error from reconstruction