The ancient Sanskrit text "Natya Shastra" by Bharata identifies and discusses twenty two "shruti" and seven shuddha and two vikrita "swara". The Natya Shastra mentions that in Shadja graama, the swara pairs saa-ma and saa-pa are samvaadi swaras (consonant pair) and are located at the interval of 9 and 13 shruti respectively. Similarly, swara pairs re-dha and ga-ni are samvaadi swara too. Without giving any example of 'a standard measure' or 'equal interval' between two successive shrutis, Bharata declared that saa, ma or pa shall have an interval of 4 shrutis measured from the pitch of the preceding swara, re or dha shall have an interval of 3 shrutis measured from the pitch of the preceding swara and ga or ni shall have an interval of 2 shrutis measured from the pitch of the preceding swara respectively. The following quote explains it all.