[iFi David]

iFi audio - The GTO filter
Part 2/4 - Introducing the iFi GTO™ Digital Filter



ALL DIGITAL FILTERS FOR AUDIO ARE WRONG.
ALL OF THEM, INCLUDING THE 'NO FILTER' OPTION.
THIS IS WHY WE NEED YET ANOTHER FILTER!

All digital filters (including no filter) differ in how they are wrong and how this influences objective measured performance as well as subjective listening performance with music and indeed, specific types of music. All digital filters add specific distortion signatures in either time vs. amplitude domain or frequency vs. amplitude domain. These distortions become all the more relevant the lower the sample rate. So, the most abundant digital music source -CD quality - is most impacted with greater possible audible consequences than High-Res content.

Wherever there is a difference, there is also a preference. Subjective listening preference may be informed by a range of factors including a learned or acquired response to recorded sound (e.g. what sounds ‘right’ or ‘hifi’ is not what sounds natural in comparison to a live performance), including direct referencing acoustic music performances.



However, with sufficient data from extensive listening tests and some inductive thinking, one should be able to propose and implement a digital filter that offers substantial improvements in removing ultrasonic noise over the ‘no filter’ (non-oversampling) case while avoiding as much as possible erring too far in the other direction with excessive and audible ringing.

So here it is - the ever so musical iFi GTO™ Digital Filter in the Pro iDSD which is the first ever seen in any DAC. In due course, technological hurdles permitting, we will try to implement it as a firmware upgrade for just about all[1] iFi audio digital products.

What is the iFi GTO™ Digital Filter?

The Gibbs Transient Optimised filter (GTO) is named after the ‘Gibbs phenomenon[2]’ in mathematics.

Wikipedia referred to the Gibbs phenomenon as “the peculiar manner in which the Fourier series of a piecewise continuously differentiable periodic function behaves at a jump discontinuity. The nth partial sum of the Fourier series has large oscillations near the jump, which might increase the maximum of the partial sum above that of the function itself. The overshoot does not die out as n increases, but approaches a finite limit.”

Most crucially, this is one cause of ringing artefacts’ in signal processing which the GTO addresses.

Way back in May 2011, the parent company of iFi audio, AMR, pioneered an earlier version of this filter in the DP-777 digital processor where it was available as an ‘Organic’ filter. Since 2011, more time has been invested into producing a filter that offered both better compatibility and technical performance than non-oversampling, while delivering a transient optimised performance that differs as little from non-oversampling as possible, delivering the new GTO™ filter.




Non-oversampling Transient response vs Organic- Digital Filter AMR DP-777

No doubt there will be extended debate if our GTO™ digital filter offers the right trade-off, compared to others. To us the two key qualities we sought was to shape of the unavoidable transient or time domain distortion so that is free of any ‘pre-ringing’ and that completes its impulse response within a fraction of the Haas (precedence effect) window; to remain in effect, inaudible to the human ear.

What we really refer to when we are talking about ringing in digital filters is actually a form of ‘Echo’ or ‘Reverb’ where, in addition to the actual transient time-shifted lower amplitude, copies of the impulse are generated using delay lines (see also the transients and digital filters section later on).

The human hearing itself is subject to an inherent transient post (impulse) ringing that completely decays within around 0.7mS[3] (see also the transients and the human hearing section later on).

The GTO filter’s transient post-ringing decays completely within 0.72mS for a 44.kHz source, ensuring that the unavoidable blurring of the transient response cannot be heard, but is integrated by the human hearing into the original transient.

This is in stark contrast to some alternative filter concepts. For example, the ‘Transient Aligned’ filter seeks a maximum number of taps, leading to an impulse response that falls well outside the Haas window. ie. its ‘ringing’ is very audible, in part because there is a pre-ringing (or pre-echo) present and in part through the sheer length of the delay line used.

For example, the 16k tap Transient Aligned filter in the Pro iDSD has an impulse response with equal pre- and post-ringing trail of around 186mS @ 44.1kHz sample rates, or a total 386mS worth of ringing. This is certainly sufficient time delay to be perceived as reverb. Using an even larger number of taps lengthens this impulse response even more.




Transient Aligned Digital Filter Transient response vs GTO™ Digital Filter iFi iDSD Pro

It may be of course, that some will prefer the sound of a very long filter, with large amounts of ringing/reverb/echo as the result is often perceived as extra added spaciousness, however, to anyone seeking to be close to the original musical performance such additives are usually unwanted.

In the end, with the iFi GTO filter, by keeping the filter short and without pre-ringing, the filter response is inaudible because it is masked by the limits of the human hearing system. At the same time this filter still permits significant attenuation of unwanted ultrasonic images, compared to non-oversampling and also other attempts at “low tap number digital filter”.

Analogy: if a 20million mega pixel camera was used to take a picture of a straight line, the naked eye would see only a straight line. As the resolution is ‘beyond’ that of the human eye, any ultra-fine imperfections are not ‘seen’. This is the same as with the GTO filter with human hearing.

If the GTO™ digital filter is so ‘perfect’, why include the other filters with the Pro iDSD? As remarked before, individual listeners may have different listening preferences and rather than imposing one option, even if we feel this option is not the best, we prefer to leave the choice down to the individual.

1) The original iDAC micro cannot receive this upgrade
2) https://en.wikipedia.org/wiki/Gibbs_phenomenon
3) “Response of the human tympanic membrane to transient acoustic and mechanical stimuli: Preliminary results” Payam Razavi, Michael E. Ravicz et al - Hear Res. 2016 Oct; 340: 15–24.

[dburna]

As someone who spent recently about a month with the Pro iDSD, I can attest that the GTO filter works....and works well. It was definitely my favorite filter of the ones resident on the Pro. I am glad to receive this information now on the basics of how it works.

Can't wait until this is available as a firmware upgrade for my iDSD Micro. I doubt I'll use any of the other filters at that point. This one just seems to have a touch more clarity to it without being harsh or etched in the least (I am very sensitive to that type of sound).

Regards, -dGB

[iFi David]

iFi audio - The GTO filter
Part 3/4 - Introducing the iFi GTO™ Digital Filter

HOW TAPS RELATES TO WHAT IS HEARD

So far, we have identified that we prefer the GTO filter because it has few taps.
Because:

More taps = more reverberation.
Few taps = minimal reverberation

Reverberation  is artificial. Sound engineers add reverb to make recordings more spacious, artificially so. Digital filters introduce reverb by the nature of their operation. In fact, a digital reverb unit operates precisely like a digital filter in principle – as depicted in this diagram.



Within digital filters are Digital Delay Lines which is defined by Wikipedia:

"A digital delay line is a discrete element in digital filter theory, which allows a signal to be delayed by a number of samples.
Delays of N samples is notated as {z} ^{-N} motivated by the role the z-transform plays in describing digital filter structures.
Digital delay lines are widely used building blocks in methods to simulate room acoustics, musical instruments and digital audio effects."

To our ears, the GTO filter simply sounds ‘right’ without any hint of artefacts or exceptional detail that feels ‘processed’, by avoiding large number of tap’s that add excessive reverb.

TRANSIENTS AND THE HUMAN HEARING

The human ear is a marvelous system with an incredible dynamic range (~135dB in middle frequencies) huge bandwidth (almost 1:1000) and a transient resolving ability that exceeds the upper limit of hearing steady state tones. Yet it is also subject to limiting factors which result in, so to speak, “blind spots” in its behavior that do not exist in purely mechanical systems (e.g. microphone). These “blind spots” can mask some behavior which objectively is distortion to be inaudible. For example, harmonic distortion masking has been well documented since the at least the 1950s if not earlier and it is reasonable to consider that ‘ringing’ on transients is also masked to a certain degree. 



If we wish to produce audio gear that is capable of operating in a way that subjectively sounds undistorted to the human hearing (the most logical preference), we must understand its limitations and capabilities. Here, we focus on the time-domain capabilities.

It has been shown that the human hearing’s time domain resolution for the initial transient may be as small as 5μs. Some debate remains as to the exact limits, though work done by Dr Peter Lennox of Derby University suggests a median between 13…18μs, or a location accuracy of less than 2 degrees.

Additionally, the transient response of the human hearing includes 500...700uS ringing caused by the ear’s mechanical system (Tympanic Membrane, Malleus / Incus / Stapes).



This ringing occurs after a transient event, there is no pre-ringing. The ringing in the ear’s system will mask any similar external ringing, which will instead be integrated into the transient, so it is inaudible.

Any pre-ringing is not masked by the human hearing, nor is any ringing that continues substantially beyond 500...700uS.

[iFi David]

Folks, RMAF!

Amazing show, amazing vibe, amazing visitors, amazing hardware! And we're there!



Many of our products are to be found at our place, including the full Pro stack (Pro iESL + Pro iCAN + Pro iDSD) and a number of fabulous headphones; Focal Utopia, Final D8000 etc.





And we have something very special in there too! The latest AMR DAC. For now we'll just put these pictures here, but in upcoming weeks we'll reveal more nfo about this machine!

[OzarkTom]

Congratulations, a rave review on the PRO IDSD  from 6moons.

https://6moons.com/audioreview_articles/ifi/

[iFi David]

Congratulations, a rave review on the PRO IDSD  from 6moons.

https://6moons.com/audioreview_articles/ifi/

Yup, we saw this and are very happy!

On a side note, it's almost holiday time...



Also we encourage you to check out our winter playlist in Tidal!

https://tidal.com/playlist/8c5a9a27-70df-4acf-8cd5-a3c938b757cf

[iFi David]

Folks, we have something short and sweet for you today. Our Thorsten talks about Pro iDSD in the context of AMR's upcoming Diablo DAC. These two machines have A LOT in common and more nfo we'll reveal shortly. In the meantime, Thorsten himself, enjoy!

https://www.youtube.com/watch?v=fl-IraSd7Zw&feature=youtu.be

[iFi David]

iFi audio – The 2018 Summary

It’s been quite the year…

Dear community,

2018 is coming to an end and we’ve decided to use this occasion to share with all of you good people a word or two. Many of our product ideas were developed from what you had to say, so we’re extremely grateful for your support. You’re the fuel to our fire, thank you!

The end of the year is also a good time to summarize what we were able to achieve in the last 12 months. Without further ado, here’s what we did:

Hardware

• March – xDSD



Before this product came to be, we knew that we had to develop something very different from what we’d done before. We thought about many things – toasters with built-in speakers, a wind-up DAC for use in the dessert…only kidding! We aimed for a portable device with wireless functionality as good as a wired connection, improved volume control and a new look. Suffice to say, we strongly believe that with xDSD we were able to deliver on all counts. The toaster has had to wait!

• May - Pro iDSD



The Pro iDSD is the most ambitious and complex product of ours to date. It started many years ago as a rather simple machine, but the more time passed, the more advanced it became. After years of suffering (and here we mean both ours AND yours, just take poor Curtis as an example), the Pro iDSD eventually came into being as a state-of-the-art desktop platform loaded with so many features that it's far easier to list things it doesn't do! Not only this, but the Pro iDSD also has A LOT in common with AMR's upcoming Diablo DAC. The former is actually a scaled down version of the latter and we're exceptionally proud of how it turned out.

• June - iFi OTG Cable



We released this item due to huge demand. It's reliable, affordable, looks sleek and there's no rocket science behind it. It does the job where it should. It's useful. Hey, if we're capable of delivering headphone amps and DACs, who said that we can't make a quality OTG cable? And nope, before anyone asks, we can’t make the equivalent Apple cable – if we could, we (and the rest of the world) surely would!!! Sadly, Apple don’t share the tech that we’d need to make one work. Would you, if you were Apple?

• June - DC iPurifier2



All our iPurifiers were developed to show that even a small and affordable item can really improve sound quality if backed by solid tech. The iPurifier2 is a step up over previous versions of this product, simply because our in-house developed circuitry got better over time. Practice makes perfect!

· July - iPurifier3.0



USB sorters have always been of utmost importance to us and the iPurifier3.0 shows our attitude towards these items. This product might look nothing like our nano or micro iUSB3.0 machines, but its roots are the very same. It was designed to audibly improve the USB interface and very positive feedback from you guys out there showed us that it does just that! Thank goodness!

• October – xCAN



Yet again X marked the spot in October! We really looked forward to releasing the xCAN. On the surface, it resembles our xDSD; they have similar set of features and they look very much alike, but the differences end there. Yes, each has finely implemented Bluetooth receiver, DAC and headphone amp in one neat enclosure BUT the xCAN is a portable and fully analogue amplifier above ALL else and its D/A conversion option is a cherry on top, whereas the xDSD - a DAC in the first place - is the exact opposite. Plus these two can work as a team and we have photographic evidence from you guys to prove it!

• November - Ear Plugs



This one you didn't expect, huh? Us neither. But then again, we like silence and it’s yet another way to give you guys a low noise floor! (Plus they can protect your ears from the squeaks of protest when your loved one finds out you’ve bought yet another piece of audio equipment!) We try to think of everything…

Software

• March - MQA firmware for all nano and micro products

This firmware was one of the most important things for us in 2018. We didn't want to include MQA functionality in just one or two products, we wanted to roll things out across all suitable iFi goods, even some discontinued models. Even though the 5.30 'Cookies & Cream' firmware has some limitations, this is the one to go after if you'd like to listen to MQA regardless of the iFi audio DAC you use.

• October - GTO filter firmware for all nano and micro products

The GTO filter was a tricky thing to develop, but in the end, it surely was worth the effort. To our ears it sounds amazing enough to surpass every other filter out there and we encourage everyone to give this one a try. Hey, it comes at nil cost, improves sound and if you don't like it, you can always use our previously released firmware. That's a win-win right there.

• November - Beta MQA firmware for the Pro iDSD

Fully operational MQA on our Pro iDSD took us a fair bit longer than expected for a reason. (Poor Curtis was turning in his grave!). The amount of work needed in order to have this signal fully decoded was nothing short of staggering. Everything had to be coded from scratch. Both our team and MQA's engineers were involved in this project and we're ecstatic that we were
able to finally have this functionality ready. Enjoy!



Additionally, one thing we’re very proud of is that we were able to push our head of sales out of the UK office for well-deserved holiday. Blindfolded, the man was in shock and had no idea where he was transported. Since no laptops or comms with his team were allowed, after the adventure he told us that withdrawal shivers kicked in hard. But once he got off the plane and acknowledged he was in Iceland… Long story’s short, after this trip he returned as a new man. Even though he’s a good lad, we didn’t miss him at all but shhh…

[iFi David]

Folks, BIG NEWS!

Good people from NativeDSD shared with us something special, to now share it with you!

The following 5 tracks in DSD1024 are available now in this dropbox folder: https://www.dropbox.com/sh/w7e9fztd56oebkm/AACoKRHGoTl5f8z18OYib1Cla?dl=0

...delivered directly by NativeDSD's mastering engineer Tom Caulfield.

• JL003 Counting Frogs
  JL009 The Sweetest Sounds
  JL002 Short People
  40318 Vivaldi - Winter II. Largo
  40719 Paganini - III. Rondo: Allegro spirituoso – Un poco più presto

These files are humongous, but surely worth the effort! Enjoy!

[FreddyJ]

Okay, so I have the AMR CD 77 - yup been around a long time - so I know who IFi. iFi sprang from AMR, right? So, I'm looking for a new DAC or DAC amp for my home set-up - my Titan amp finally gave up. Anyway - why should I choose the Pro iDSD? I know I can read all the reviews but give it to me in one paragraph - clean and simple? Thanks guys.

[iFi David]

Pro iDSD is a fabulously musical DAC. And a headphone amp. And a preamp. And a streamer. And it works with pendrives loaded with music. And it has both transistors and tubes inside. And it has a bunch of digital filters to play with. And it upsamples music up to DSD1024. And... it does it all. Literally.

[Tyson]

Pro iDSD is a fabulously musical DAC. And a headphone amp. And a preamp. And a streamer. And it works with pendrives loaded with music. And it has both transistors and tubes inside. And it has a bunch of digital filters to play with. And it upsamples music up to DSD1024. And... it does it all. Literally.

Also, it sounds better than my prior reference DAC for less than $5k - the Auralic Vega.  Which I owned prior to buying an iDSD Pro.

[iFi David]

 Folks, Pro iDSD Firmware v2.08 - now live on the website - https://ifi-audio.com/downloads/

[iFi David]

Self-taught acoustic guitar singer-songwriter David Elias lives in Hawaii. He has played music all his life. He has written songs since earliest teen years.

He became an adventurous musician online in the earliest Internet days and began self-producing a seemingly endless menu of music and styles, settings, backing bands and artists on his own label Sketti Sandwich Productions.

In the early 2000's he helped pioneer some of the first hi-res technologies (DSD) to create the most realistic listening reproductions for listeners in both stereo and surround sound as if you are in the studio with the artists performing.

Read what David has to say about iFi's Pro iDSD: https://art-of-listening.com/…/the-ultimate-media-refactor…/

[iFi David]

Is Pro iDSD and Pro iCAN the best amp/DAC stack out there? I have no idea, but Joshua Valour addressed this question in truly great fashion. Please see this vid!

https://www.youtube.com/watch?v=4vwscVmZLsg&t=122s