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Topic: The Active High Frequency Solution - AHFS

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Posted by Romy the Cat on 02-06-2008

 Romy the Cat wrote:
Anyhow, I would like to add some addition to my “to do” list for 2008.

8) Try my new experimental AHFS in context of my RAAL “Water Drop”

OK, I would like to share with readers of my site a notion is that I am would like to look into. First think first let defend I call tweeter from now and below. A tweeter is something that use above I would say 10-12kHz. So, it you tweeter is crossed at 2.100Hz or 1.800Hz then do not read further - it would have littlie relativity to you.

I have notice is that in most oaf of the cases the high frequency solutions for our acoustic systems have inherent problems. The patterns with which our tweeters react to musical events have different impact my listening consciousness. It does depend in a way from the type and topology of tweeters used but still something fundamentally wrong with tweeters generally. I have no problems to resolve all tweeters problems (time alignment, all aspects of integration and etc …etc etc …) but still the tweeters jump out from what I feel is right presentation of sound. It is not my tweeter. I have good tweeter and it is very smartly used. The problem I report is applicable for all tweeters I have heard, including my tweeter.

So what it is. The problem that I see is a certain dynamic non-linearity of tweeters. A properly calibrated and integrated tweeter in some cases runs too hot during soft passages and some time too hot during the loud passages. When I am taking about too hot I do not really mean “bright” but rather a very minor deviation of what I would call proper balance. The proper balance could be evaluated not but by “sound too hot” or “not enough air” but rather by assessing the relationship between HF Spaces and MF space. Well, if the playback properly performs full-range and properly calibrated then it is possible to assess the relationship between HF Spaces and LF space but it is more complicated for most people as there are many “issue in this subject”.

Anyhow, I was trying to catch the pattern according to which tweeters do dry too fast of over spit the MF and I was not able to. That leaded me to invent a concept of “Active High Frequency Solution” – AHFS. This is purely conceptual invention and I have no specifics. The AHFS is a concept what a tweeter has own brain and can move own output in consistence with what MF channel does. I am talking an intelligent automated compressor/extender/volume control/harmonic fertilizer that execute some king of God know what kind parabolic or hyperbolic algorithm and changes the volume and harmonic content of tweeter in dependency of what HF and what MF do. Hypothetically I can see a mechanism, fed from HF and MF chanals, which in real time (1/10 of the tweeter rate) changes gain and loading of the output tube that drive tweeter. It is not so hard to implement – what hard is to know how to process and interpret the needs to this active non-linear correction.

I share this idea with some folks wondering what they feel about it. No one proposed any intelligent explanation or was able to collaborate the subject. No one except the RAAL’s Alexander Radisavljevic. Alex proposed a very interesting idea what it might come from. He suggested that the microphone compressors that pro flasks use mostly have limited compression bandwidth and they stop compression at 12-14kHz. The tweeters and particularly his ribbons are dynamically –linear at HF and therefore they just over-dynamic above 12-14kHz. I feel that it might be very plausible direction to think as I did detect that the tweeters that have own compression at higher frequencies (fabric tweeter) practically immuned from the problem. I was intentionally listening today of some recordings that use no compression of any kind and I realized that the problem is not there even with my tweeter. So, would it be that my tweeters have a need to recognize the type of the compression that was used during the recording, detect the excessive dynamic of the HF ingredient and then to implement in a real time a contra-measure to the problem?

Well, sometime I would like to attack this notion and try to execute some form of Active High Frequency Solution. If you, the readers, have any ideas, thoughts or initiatives on the subject that feel free to share.

Rgs, Romy the Cat
Posted by Paul S on 02-07-2008
From Alex's thoughts it seems like one approach might be related to/interfaced with the program itself, such as RIAA or FFRR, etc., etc., even though I think those "recommended guidelines" have been only loosely observed by recording engineers, who have tended to pretty much do as they pleased, session to session and track to track, all along.

Anyway, my own observation is that this problem can start as low as, perhaps, 7k Hz.  I think this is why I "liked" the "good" silk dome tweeter set to run itself out on top, overlaid with an electrostatic array entering slowly from about 7.5k Hz.  This never"solved" the problem, but it did +/- temper the aggression from the worst "curves" in the recordings, if at the expense of true HF capability.

It seems like there has always been a problem trying to cross over between the texture and "traction" of a decent dynamic "tweeter" and the only acceptable true HF for me at this point, the naked ribbon.  Each ultimately exposes the other's weaknesses to the acute listener, at least relative to what "might" be.

Another annoying thing about the HF ribbons is their extreme sensitivity to... everything.  They hate the crossovers that they absolutely must have and they can make good amps sound mediocre.  With bad electricity it would be nice to be able to just throttle them back or even shut them down entirely.

Alex is way past smart; so I have to keep reminding myself that he likes to run his ribbons way down there, which I just do not get.

Oh, well...

Ribbons: Can't live with 'em; can't replace 'em.

I think we have already started the idea of the general up-tilting balance of hi-fi, and I think this may also figure in to this discussion.

Best regards,
Paul S
Posted by el`Ol on 02-08-2008
In my opinion the trend towards AMTs in mainstream audio ist justified. They integrate very well with direct radiator midranges, even when they are crossed low. They are more dynamic and detailed than fabric domes, but not absolutely unforgiving. The German distributor of RAAL announced that a German manufacturer will be present with the large RAAL at the next Highend show. I fear it is Ascendo. They use to be present there with their silly low-crossed two-way speakers. I hope I am wrong.
Posted by Romy the Cat on 02-08-2008
I am not quite sure why Paul came up in this thread with the subject of “ribbons sensitivity to... everything”. That is absolutely not relevant to the AHFS concept. el`Ol, the ribbons and AMT tweeter I feel absolutely identically wrong if to approach them from the needs a tweeter to demonstrate the AHFS behavior. They both have static mistakes, the mistakes that are described by the design of each transducer. Ribbons it looks like too AHFS-transparent and have no idea where to negotiate own dynamic capacity. AMT drivers, with the transducers cocooned into a plastic film, generally are too slow and too dynamic ly-colored. The most important is that this dynamic coloration is very static and presented at each recording identically. So, in context of this thread it should not be “AMT vs. ribbon” but rather the AMT and ribbon vs. AHFS.

The Cat
Posted by Paul S on 02-08-2008
Romy, of course I frame my remarks according to how I "understand" the issue at a given point, ie., dialog.  If your response helped me to undersatnd your idea of this problem any better, then it seems to go back to the texture/traction issue in part, at least in the sense that I think this is the ribbon's (and other true HF) main shortcoming and perhap the main reason for the "dynamic stasis" at frequencies only the naked ribbon can otherwise render successfully, as far as I know.

NOT saying I grasp your notion entirely even now, but I hear drivers that have better dynamics simply crapping out at HF, and the ribbons or 'stats simply failing to match the best dynamic drivers in that sense.  Right or wrong, I have always figured this had to do with the physical design limits of the specialized drivers.  It never occurred to me that there might be a way to electronically compensate for the lack of grunt on the part of the very delicate drivers that are made the way they are.

NOT saying it is so, but just to try to get a better handle on your concept, do you think a hypothetical "super tweeter" MIGHT possibly solve the HF dynamics issue by having more grunt available when it was called for by the music?  Or can the problem you speak of ONLY be addressed via electronics and some sort of feedback/reaction sequence?

Best regards,
Paul S 
Posted by Andy Simpson on 03-02-2008

Romy,

In all your search for better sound quality, did it ever occur to you that the greatest time-domain distortion might occur at the other end of the signal chain?

The problems you perceive at the upper end of the spectrum are as likely problems with the microphones used in the recording as with your playback setup.

Many people do not understand the scale of the issue. Small resonance (time-domain) problems in microphones become big problems on playback.

If you are making critical decisions about your playback system according to your impressions when listening to a recording - you had better be sure of the equipment used.

To me, it sounds like you are describing the sound of the condenser microphone (especially the measurement variety).

Don't under-estimate the lack of listening that goes into the design of the average microphone.

Most of the designers have never heard a horn system and are no more critical of the direct-radiator speakers than they are of their own microphones.

Andy

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