I have mentioned that I believe SST to be already highly uniform, and that this uniformity has somehow been involved with the spread of mass-media forms that use it, such as radio and film. Impressionistic accounts attribute the development of this inter-caste, interregional form as taking place in college hostels when young educated people from all over TN come together and must negotiate some form of communication. The inter-caste inter-regional form used to be the Brahman dialect, but this is no longer the case; now even Brahmans use NBr. Tamil, and clearly SST has evolved out of this panlectal NBR dialect soup.
I have also used terms like ``SST does not allow form x" or ``when in doubt, SST prefers forms close to LT" and with such locutions I have been speaking as if SST were a person or a decision-making body. In fact the decision that went into the choice of this form or that form are covert, i.e. they are not available for observation, but anecdotal reports from speakers who have learned their SST in college hostels confirms that a kind of decision-making process goes on. Certain forms are stigmatized, e.g. Brahman forms, so Brahman speakers quickly learn not to use their home dialect, if they have not already figured this out. Other speakers may bring regional or caste forms to the process, only to have them stigmatized through ridicule and other forms of overt comment; they quickly learn to not use these forms again. If this business sounds familiar, it is probably because a similar process seems to have evolved in English public schools in the 18th and 19th century, whence the ``standard" Received Pronunciation (RP) evolved.
The interesting thing in this decision-making about what is an acceptable SST form and what is not, is that it is not governed by rules set by an Academy, by lexicographers, by eminent writers, or any of the other elite language control boards found in many societies, e.g. the French Academy, the Duden Gesellschaft (for German), etc. Yet college students are an elite, and they have in common that they are or were (I do not have a date for the evolution of this NBr. SST) educated, either in English or in Tamil Originally then the body of people who made the decisions were most likely to have been male, of higher non-Brahman castes, and from families wealthy enough to afford higher education of the western type. This is of course, not at all unlike the situation applying in the British RP model.
In the mid-twentieth century, it is without question that the chief disseminator of this SST has been the modern Tamil ``social" film. There is remarkable uniformity of SST irrespective of whether the studios were DMK-dominated or Congress-dominated, i.e. MGR films vs. Sivaji Ganesan films, to take only two examples. Despite the DMK's special ideas about Tamil, their films used SST that varied hardly at all from the kind found in other studios' films, except when the hero feels the overwhelming urge to expatiate in the special DMK-preferred alliterative style. This variety is also found in the stage dramas of the social variety that in fact have a symbiotic relationship with the Tamil film industry, and is also used in radio plays, and to a lesser extent in television. Another place where some kind of SST is also used, but with less consistency, is in the so-called ``social" novel and short story. Here writers are involved, but not as prime movers in the decision-making process.
Beginning with the advent of novel and short-story writing in Tamil, there evolved a kind of writing that was concerned with social problems, moral uplift, the independence movement, and other social issues brought on by the collision of colonialism with traditional India. This kind of prose-writing did not actually exist before, nor did almost any kind of prose--everything in Indic languages tends to be in rhymed sutras, more suitable for memorization. In order to make these writings appear to reflect the lives of real people, writers began to use some spoken styles in the dialogues of their writings. Never, to my knowledge, or perhaps very rarely, was a novel/short story written totally in a spoken style. The narrative and descriptive portions of the novel are always written in a form that I would call modern Literary Tamil, which does not admit most of the spoken changes that have occurred since the 13th century, but is more relaxed about, e.g. sandhi rules, than would be older forms.
However the spoken styles are not perfect examples of spoken Tamil, i.e. we cannot use them as true phonetic renditions of how people actually spoke, because there are a number of inconsistencies in this use.