Arun,
Some thoughts on the write up...
http://arunk.freepgs.com/blog/bharatas_22_sruthis.html
Instead the shadja and rishaba are consonant.
Typo - should read "Instead, rishaba and panchama are consonant".
I am actually puzzled at how Bharata or his contemporaries chose the pitch for rishaba and dhaivata as they do not have consonant relationship (i.e 4/3 or 3/2) with other svaras in shadja grama
My speculation: it may stem from the choice of 5/3 for dhaivata. While the ratio 5/3 may not be termed a strict "consonance", it is weakly consonant and after some practice very clearly so. From my experience, if I were to create a hierarchy of consonances, from the easiest to the most difficult, it would be 3/2 (P), 4/3 (M1), 5/4 (G3), 5/3 (D2) and 9/8 (R2). With training, very accurate ear tuning can be done for all of these consonances. It may come as a surpise to some that Sa-Pa is slightly easier to tune than Sa-ma. For example, vina players interesting in tuning well would be well advised to tune their second string Pa after first tuning their third string (lower Sa) and then tuning the pa with respect to that lower sa.
I will try to dig up the psychoacoustics of non-culture based consonance perception. I suspect some of it is simply the odd harmonics inherent in a vibrating string... for example, pa (3/2, third harmonic) and G3 (5/4, fifth harmonic) and R2 (9/8 ninth harmonic).
Coming back to your "puzzle", therefore Bharata may have established 5/3 for dhaivata based on weak consonance with shadja and then fixed 10/9 as a strong consonant of that for rishaba...As an aside, I am convinced that our current fixation with 9/8 stems from using panchama in the aadhara shruti. Just using a Sa-only tampura somewhat removes the 9/8 fixation since the ninth harmonic is not readily discernible (in a tampura with Pa, the 9/8 stands out prominently as the third harmonic of the Pa string). I'm trying out tampura with sa-only as well as 10/9 for khpriya and it is really pleasant. Will post a clip soon.
For my part, I was wondering why Bharata (or whoever, BTW, my only reference to Bharata is your write up!) would declare consonance between R and D as an axiomatic "rule" for the shadja graama. For this I have a speculation from my own tuning practice, based on pleasantness, for harikamboji scale. What I use ends up being a madhyama mUrchana of the shadja grama. The shadja grama is 1, 10/9, 32/27, 4/3, 3/2, 5/3, 16/9,2 and its madhyama murchana would be the beautiful harikamboji scale 1, 9/8, 5/4, 4/3, 3/2, 5/3, 16/9 and 2. What's interesting is that 9/8 seems appropriate for all the "major" scales (hari, shank, kalyani) and 10/9 for all the "minor" scales (khpriya, natabh). Also, when playing a raga like kamboji (derivative of harikamboji) one can explore the vadi-samvadi relationship of the phrases ND and MG very beautifully. As I understand Harikamboji was a prominent scale at some early time ??
I think that When thinking about gramas and tuning practices one must think concurrently about murchanas and graha bhedas and then find out the best combination of consonances that would work out for a variety of graha bhedas. In this shadja grama, some other murchanas are:
Ri - 1, 16/15, 6/5, 27/20, 3/2, 8/5, 9/5, 2 (todi with slightly different "ratios" than we might try to use today)
ga - 1, 9/8, 81/64, 45/32, 3/2, 27/16, 15/8,2 (kalyani with slightly different "ratios" than we might try to use today)
ma - 1, 9/8, 5/4, 4/3, 3/2, 5/3, 16/9, 2 (harikamboji, perfect fit)
pa - 1, 10/9, 32/27, 4/3, 40/27, 128/81, 16/9 (natabhairavi with a lowered panchama)
da - nothing useful because there's no panchama
ni - 1, 9/8, 5/4, 4/3, 3/2, 27/16, 15/8, 2 (almost perfect shankarabharanam, higher dhaivata than we might use today)
Note that with the exception of "natabhairavi" the panchamas of all the murchanas fit perfectly. I know I am using current raga names when discussing a very early phase in the development of music. The names are only a place-holder since I don't know much history

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Now obviously all the above ratios are quite intimidating and might seem nonsensical
Did you examine the possibility that ALL the strings of the chala vina need not be decremented by the SAME amount every step. After all the process is not purely about strict mathematical ratios. It is about practical stuff. Also Bharata does not seem to have been aware of the different "shades" of his "pramana" shruti - 81/80, 25/24, 256/243. Therefore, one can postulate the following (implicit in Bharata):
"At each step, every svara of the chala vina is decremented by one pramana shruti which can take values of 81/80, 25/24 or 256/243 on a svara by svara basis"
Did you examine if this would make some of the ratios less daunting, although I agree that they are quite irrelevant. I am too lazy to work out the math and combinations

. The only problem here may be that while we have implicitly assumed that the shadja grama can be tuned by some prior art (which is not a bad assumption, as even I can do it myself based on a variety of consonance sequences), based on fixing ANY one note, we don't know if these alternating scenarios can be tuned at all ! After all, it is complete idiocy to presume that somebody can reduce a string reliably by one pramana shruti. Without a consonance, one cannot even be sure of Sa-pa-sa leave alone a pramana shruti lower. So there goes that theory

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but there does not seem a strong enough rationale then to claim that an intervening sruthi with a ratio like 256/243 yielded via such a division is singable and is indeed sung today. One could argue that it is based on an inaccurate interpretation of Bharata's division to express a pitch that is perhaps perceived as lower than typical mayamalavagowla rishaba (the gowla rishaba is sung as a pitch inflexion from shadja)
Folks like Arvindh have analyzed current Carnatic practice very well but might also have thrown a small baby with the bathwater

. What is "singable" really depends on the level of training and commitment. For example, nobody would dispute that the ratios Sa (2) and pa (3/2) are "singable". And yet, I can cite umpteen practitioners of Carnatic music, right up to the present day, who sometimes find those notes not so "singable"

. Does this mean they cannot do it ? Maybe they aim for one thing, and end up elsewhere due to physiological and other limitations. Therefore, with sufficient training it is possible to "aim" at different sounds (i.e., ratios) and achieve success depending on the factors cited. The bigger question, whether it has any impact on the listener, is also not easily answerable. For example, when MMI or KVN or MSS sing, some people find the "purity" exhilarating. At the bottom of this "purity" lies one predominant fact - they knew what ratios to "aim" for and consistently achieved a very high statistical hit rate. So even if an ordinarly listener cannot empathize with the aims and success rate, they might discern some special "purity". Now is this "purity" the only thing in CM ? Absolutely not. Semmangudi and GNB sometimes aimed poorly and yet continued to thrill audiences.