My native English accent is the General American accent (GAm), which is the standard American English accent.  Received Pronunciation (RP) is regarded as the standard accent of Standard English in the United Kingdom. 

Comparison of General American and Received Pronunciation:

GAm is rhotic while RP is non-rhotic; /r/ is only pronounced in RP when it is immediately followed by a vowel sound. Where GAm pronounces /r/ before a consonant and at the end of an utterance. 

RP has 3 open back vowels, where GAm has only 2 or even one. Most GAm speakers use the same vowel for RP "short O" /ɒ/ as for RP "broad A" /ɑː/ (the father–bother merger); many also use the same vowel for these as for RP /ɔː/ (the cot–caught merger).

RP has a marked degree of contrast of length between short and long vowels..In GAm this contrast is much less evident.

In GAm flapping is common: when either a /t/ or a /d/ occurs between a sonorant phoneme and an unstressed vowel phoneme...This sounds like a /d/ to RP speakers, although many GAm speakers distinguish the 2 phonemes by aspirating /t/ in this environment, especially after /ɪ/ or /eɪ/ (thus bitter and rated are distinguishable from bidder and raided), or by lengthening the vowel preceding an underlying /d/. ...Pronouncing the t is often considered overly formal. The degree of flapping varies and is often reduced in more formal settings. This does not mean it always completely merges with bedder, as many speakers enunciate the d so as to distinguish it slightly from the flapped t."

<a href="http://en.wikipedia.org/wiki/Comparison_of_General_American_and_Received_Pronunciation">http://en.wikipedia.org/wiki/Comparison_of_General_American_and_Received_Pronunciation</a>;

Yes, you pasted a similar explanation before.  I just wanted to make it clear that there is no standard dialect or accent of American English.  In fact, the USA does not even have an official language.

Hello,  Thaddeus.   "  General American (GA), also known as Standard American English (SAE), is a major accent of American English.  

General American, like British Received Pronunciation (RP) and most standard language varieties of many other societies, has never been the accent of the entire nation. However, it has become widely spoken in many American films, TV series, national news, commercial ads, and American radio broadcasts.  ..... 

The General American accent is most closely related to a generalized Midwestern accent and is spoken particularly by many newscasters. .... "

<a href="https://www.princeton.edu/~achaney/tmve/wiki100k/docs/General_American.html">https://www.princeton.edu/~achaney/tmve/wiki100k/docs/General_American.html</a>;

There is no standard American accent.  The term General American English is a sociolinguistic observation based upon how many newscasters speak to a national audience.  There are no federal mandates regarding what accent/dialect should be taught in schools.  We lack any kind of official academies or official institutions for determining and regulating a standard.

There is a YouTube video of a Londoner who amazingly speaks the English language in 24 different accents.

"The English Language In 24 Accents"

<a href="http://www.youtube.com/watch?v=dABo_DCIdpM">http://www.youtube.com/watch?v=dABo_DCIdpM</a>;

视频: The English Language In 24 Accents

<a href="http://v.youku.com/v_show/id_XMjExNzU0MDU2.html">http://v.youku.com/v_show/id_XMjExNzU0MDU2.html</a>;