I agree with you on this: sa as in samskritam came into usage only of late.
Thanks for your confirmation Arasi. I have heard malayalees pronounce aracan with the ca sound (note the ca sound is close to the original ja of rAjan) which was the main purpose of substituting the foreign ja with the native ca.
Talking of mispronounciation, கிழவி often becomes கிளவி to many Tamils. Most Tamils today cannot distinguish between dentals and alveolars (ர vs ற, ந vs ன etc) except to state generally on the basis of familiarity that the alveolars cannot be used to start a word etc.
unlike ச which can adopt ca/sa depending on context similar to other vallinams
Your assumption that early tamil allowed ca to be allophonic to sa is not borne out by the facts below. Else it would have converted all sa sounds of sanskrit to ச and would not have simply dropped the sa sounds from sanskrit words.
Apart from that the voiced allophones are like this:
ka - ga,
ca - ja, ta - da, pa - ba
Where does sa come into the picture at all? sa is not the voiced variant of ca, ja is.
See the aracan example I've given above where rA-ja-n becomes ara-ca-n (if the original word had been rA-sa-n, Tamil would have dropped the sa and converted the word to aran). Other dravidian languages employed other devices to deal with the sanskrit ja. FOr example in kannada/telugu rAja became rAya, like krishna-deva-rAya (compare these with english words like royal, french roi etc which underwent similar changes under entirely different circumstances).
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Coming back to the issue of sibilants in Tamil, the language took great pains in the past to remove sibilants even from sanskrit words before absorbing them.
Many Tamil people pronounce the english word school as "iskool", a Tamil word cannot start with a s sound (even for a borrowed word like school). So please note:
summA
summA
solla mudiyaathu.
Tamil did not allow borrowed words to contain 's' sound even in the past; it first simplifies sahasram to sa-ha-si-ram and finally sheds the s and h sounds to form a-a-i-ram = aayiram.
The word stuti becomes tuti (தà¯à®¤à®¿) and correspondingly stotram became தோதà¯à®¤à®¿à®°à®®à¯. It even converted AkAsha (sky) to AkAya (ஆகாய), rA-kSa-sa became a-ra-kka dropping the last sa and converting the kSa to kka.
Similarly the sanskrit word sa-bh-A is turned into a-v-ai = avai by shedding initial s sound.
Again consider the month name sr-AvaN-a, where the word becomes AvaN-i in tamil by shedding initial "sr-". Malayalam further changes the Ava (a-u-a) into O and the name becomes ONam
Yet again consider the name of the star sr-a-viSTam, in tamil it becomes a-viTTam where the "sr-" is lost (since in Tamil a word can neither begin with an s or an r) and the complex syllable STa is simplified into TTa.
Again consider the month name A-sh-A-Dha which sheds the sh sound, and becomes A-A-Di in Tamil.
The sanskrit word for month - mA-sa is also changed to mA-ta in standard tamil.
Similarly the name of the month TaiSi becomes Tai in Tamil dropping the last Si.
Also the month mArga-shI-rSa drops the medial shI and becomes mArga-zhi in Tamil, the change to Tamil's retroflex approximant zha is apparently facilitated by the retroflex fricative occuring in the rSa syllable.
So I have shown that Tamil removes sibilants from sanskrit words whether it occurs as the first letter, somewhere in the middle or at the final syllable of the word.
chandra however becomes chandira (சநà¯à®¤à®¿à®°) because the ca is not a sibilant like sa/Sa/sha. Similarly the word chandana in sanskrit stays the same in Tamil as சநà¯à®¤à®©à®®à¯, since it has the ca sound and not the sa sound. The word vrischika becomes விரà¯à®šà¯à®šà®¿à®• in Tamil (only ch sound remains shedding the s). So Tamil has in the past accepted ca (ச) sounds which already exists in Tamil, not Sa/sha/sa sound which are alien to Tamil.